Semiconductor device and manufacturing method therefor

ABSTRACT

An active layer of an NTFT includes a channel forming region, at least a first impurity region, at least a second impurity region and at least a third impurity region therein. Concentrations of an impurity in each of the first, second and third impurity regions increase as distances from the channel forming region become longer. The first impurity region is formed to be overlapped with a side wall. A gate overlapping structure can be realized with the side wall functioning as an electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having circuitsstructured with thin film transistors (hereinafter referred to as TFT).For example, the present invention relates to electro-optical devices,typically liquid crystal display panels, and to the structure ofelectronic equipments loaded with such electro-optical devices as parts.Note that throughout this specification semiconductor device generallyindicates devices that acquire their function through the use ofsemiconductor characteristics, and electro-optical devices,semiconductor circuits, as well as electronic equipments aresemiconductor devices.

2. Description of the Related Art

Active matrix type liquid crystal display devices composed of TFTcircuits that use polysilicon films have been in the spotlight in recentyears. They are the backbone for realizing high definition imagedisplays, in which a plurality of pixels are arranged in a matrix state,and the electric fields that occur in the liquid crystals are controlledin that matrix state.

With this active matrix type liquid crystal display device, as theresolution becomes high definition such as XGA and SXGA, the number ofpixels exceeds one million. The driver circuit that drives all of thepixels is extremely complex, and furthermore is formed from a largenumber of TFTs.

The required specifications for actual liquid crystal display device(also called liquid crystal panels) are strict, and in order for all ofthe pixels to operate normally, high reliability must be secured forboth the pixels and the driver circuit. If an abnormality occurs in thedriver circuit, especially, this invites a fault called a line defect inwhich one column (or one row) of pixels turns completely off.

However, TFTs which use polysilicon films are still not equal to theMOSFETs (transistors formed on top of a single crystal semiconductorsubstrate), used in LSIs etc., from a reliability point of view. As longas this shortcoming is not overcome, such a view that it is difficult touse TFTs when forming an LSI circuit gets stronger.

The applicant of the present application considers that a MOSFET hasthree advantages from a reliability standpoint, and infers the reasonthereof as follows. A schematic diagram of a MOSFET is shown in FIG. 2A.The MOSFET contains a drain region 201 formed on a single crystalsilicon substrate, and an LDD (lightly doped drain) region 202. Inaddition, there is a field insulating film 203, and a gate insulatingfilm 205 directly under a gate wiring 204.

In that arrangement, the applicant considered that there are threeadvantages from a reliability standpoint. The first advantage is animpurity concentration gradient seen when looking at the drain region201 from the LDD region 202. As shown in FIG. 2B, the impurityconcentration gradually becomes higher from the LDD region 902 towardthe drain region 201 for a conventional MOSFET. This gradient isconsidered effective in improving reliability.

Next, the second advantage is that the LDD region 202 and the gatewiring 204 overlap. Known examples of this structure include GOLD(gate-drain overlapped LDD), LATID (large-tilt-angle implanted drain),etc. It becomes possible to reduce the impurity concentration in the LDDregion 202, the relaxation effect of the electric field becomes larger,and the hot carrier tolerance increases.

Next, the third advantage is that a certain level of distance exists inbetween the LDD region 202 and the gate wiring 204. This is due to thefield insulating film 203 being formed by a shape in which it is slippedunder the gate wiring. Namely, a state in which only the overlappedportion of the thick film gate insulating film becomes thick, so aneffective relaxation of the electric field can be expected.

A conventional MOSFET compared with a TFT in this way has severaladvantages, and as a result, is considered to possess a highreliability.

In addition, attempts have been made in which these MOSFET advantagesare applied to a TFT. For example, Hatano et al (M. Hatano, H. Akimoto,and T. Sakai, IEDM97 Technical Digest, p. 523-526) realized a GOLDstructure that uses sidewalls formed by silicon.

However, compared with a normal LDD structure, the structure publishedin the paper has a problem in that the off current (the current thatflows when the TFT is in the off state) gets large, and therefore acountermeasure is necessary.

As described above, the applicant of the present invention considersthat, when the TFT and the MOSFET are compared, the problems associatedwith a TFT structure affect its reliability (especially its hot carriertolerance).

SUMMARY OF THE INVENTION

The present invention is technology for overcoming this type of problem,and therefore has an object of the invention to realize a TFT thatboasts the same or higher reliability than a MOSFET. In addition,another object of the invention is to realize a semiconductor devicewith high reliability which includes semiconductor circuits formed bycircuits using this type of TFTs.

An active layer of the NTFT of the present invention is firstlycharacterized by including three impurity regions, other than a channelforming region, which have at least three different impurityconcentrations. With this, an LDD structure can be obtained, in whichthe impurity concentration becomes gradually higher away from thechannel forming region (in proportion to the distance from the channelforming region). Namely, it is possible to increase the TFT'sreliability by a relieved electric field at the drain edge (vicinity ofthe border between the drain and the channel forming region).

An aim of the inventor of the present invention is to intentionally forma plurality of regions with the concentration gradient of the LDDsection of an exemplary, conventional MOSFET. Therefore, there is noproblem with the existence of three or more impurity regions.

Further, a second characteristic of the present invention resides inthat it is formed into a state in which the gate wiring (including thegate electrodes) covers (overlaps) at least a part of the LDD region,through the gate insulating film. Deterioration due to a hot carrier canalso be effectively suppressed with this type of structure

In addition, a third characteristic of the present invention is that,through the multiplier effect of the first characteristic and the secondcharacteristic described above, the reliability of a TFT can be raised.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a cross section of a CMOS circuit of thepresent invention;

FIGS. 2A and 2B are drawings showing the structure of a cross section ofa conventional MOSFET;

FIGS. 3A to 3E are drawings showing the manufacturing step of a CMOScircuit of Embodiment 1;

FIGS. 4A to 4D are drawings showing the manufacturing step of the CMOScircuit of Embodiment 1;

FIGS. 5A and 5B are drawings showing the manufacturing step of apolysilicon film of Embodiment 1;

FIGS. 6A and 6B are drawings showing the manufacturing step of thepolysilicon film of Embodiment 4;

FIGS. 7A and 7B are drawings showing the manufacturing step of thepolysilicon film of Embodiment 5;

FIGS. 8A to 8D are drawings showing the manufacturing step of the CMOScircuit of Embodiment 7;

FIGS. 9A to 9D are drawings showing the manufacturing step of the CMOScircuit of Embodiment 8;

FIGS. 10A and 10B are drawings showing the manufacturing step of theCMOS circuit of Embodiment 9;

FIGS. 11A and 11B are drawings showing the manufacturing step of theCMOS circuit of Embodiment 11;

FIG. 12 is a drawing showing the external appearance of anelectro-optical device of Embodiment 16;

FIGS. 13A to 13D are drawings showing electronic equipments ofEmbodiment 18;

FIG. 14 is a drawing of a CMOS circuit viewed from a top surface of thepresent invention;

FIGS. 15A to 15C are drawings showing the structure of a pixel matrixcircuit of Embodiment 12;

FIG. 16 is a drawing showing the structure of the pixel matrix circuitof Embodiment 13;

FIG. 17 is a drawing showing the structure of the pixel matrix circuitof Embodiment 14;

FIG. 18 is a drawing showing the structure of the pixel matrix circuitof Embodiment 15;

FIGS. 19A to 19H are drawings for comparing several types of TFTstructures of the present invention;

FIGS. 20A and 20B are drawings showing the energy bands for an NTFT(off-state) of the present invention;

FIGS. 21A to 21E are drawings showing the manufacturing step of the CMOScircuit of Embodiment 19;

FIGS. 22A and 22B are drawings showing the manufacturing step of thepolysilicon film of Embodiment

;

FIGS. 23A to 23D are drawings showing electronic equipments ofEmbodiment 18;

FIGS. 24A to 24D are drawings showing electronic equipments ofEmbodiment 18;

FIGS. 25A-25B are drawings showing the outline of an EL display panel ofEmbodiment 20;

FIG. 26 is a drawing showing the cross section of the pixel portion inthe EL display panel of Embodiment 20;

FIGS. 27A-27B are drawings showing the top view of the pixel portion inthe EL display panel and the circuit structure for the pixel region,respectively of Embodiment 20;

FIG. 28 is a drawing showing the cross section of the pixel portion inan EL display panel of Embodiment 21;

FIGS. 29A-29C are drawings showing different circuit structures forpixel portions in EL display panels of Embodiment 22;

FIGS. 30A-30B is drawings showing the outline of an EL display panel ofEmbodiment 24;

FIGS. 31A-31B is drawings showing the outline of an EL display panel ofEmbodiment 25.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment mode of the present invention is explained with referenceto FIG. 1. Note that a cross sectional view is shown in FIG. 1, and thata view looking from above is shown in FIG. 14. In FIG. 1, referencenumeral 101 denotes a substrate having an insulating surface. It ispossible to use, for example, a glass substrate with a prepared siliconoxide film, a quartz substrate, a stainless steel substrate, a metallicsubstrate, a ceramic substrate, or a silicon substrate.

A characteristic of the present invention resides in the structure of anactive layer in an N-channel type TFT (hereafter referred to as NTFT).An NTFT active layer is formed by including a channel forming region102, a pair of first impurity regions 103, a pair of second impurityregions pair 104, and a pair of third impurity regions 105. Note thatthe impurities doped into each of the impurity regions are elementsbelonging to the group 15 of the periodic table (typically phosphorousand arsenic).

The channel forming region 102 at this time is either an intrinsicsemiconductor layer, or a semiconductor layer which has been doped withboron to a concentration of 1×10¹⁶ to 5×10¹⁸ atoms/cm³. Boron is animpurity used to control the threshold voltage and to preventpunch-through, but other elements can be substituted if the similareffects may be provided. The doping concentration in that case issimilar to the level for boron.

Note that the semiconductor layers that can be used by the presentinvention include not only semiconductors with silicon as their maincomponent, such as silicon, germanium, and silicon germanium. Chemicalcompound semiconductor layer such as gallium arsenide can also be used.In addition, the present invention is applicable to TFTs that useamorphous semiconductors (amorphous silicon etc.) as the active layer,as well as to TFTs that use semiconductors that include crystals(including single crystal semiconductor thin films, polycrystallinesemiconductor thin films, microcrystalline thin films).

Further, each of the first impurity regions 103 on the NTFT has a lengthof between 0.1 to 1 μm (typically between 0.1 to 0.5 μm, preferablybetween 0.1 to 0.2 μm), and includes a concentration of a periodic tablegroup 15 element (phosphorous is typical) in the range of 1×10¹⁵ to1×10¹⁷ atoms/cm³ (typically between 5×10¹⁵ to 5×10¹⁶, preferably between1×10¹⁶ to 2×10¹⁶ atoms/cm³). Note that this impurity concentration isdenoted as “n⁻” (the “n⁻” region refers to the first impurity regions103 in this specification).

Also note that throughout this specification, unless otherwiseparticularly specified, “impurity” is used to specify either a periodictable group 13 element or group 15 element.

Each of the second impurity regions 104 has a length of between 0.5 to 2μm (typically between 1 to 1.5 μm), and includes a concentration of aperiodic table group 15 element in the range of 1×10¹⁶ to 1×10¹⁹atoms/cm³ (typically between 1×10¹⁷ to 5×10¹⁸ atoms/cm³, preferablybetween 5×10¹⁷ to 1×10¹⁸ atoms/cm³). It is desirable to regulate theimpurity concentration in the second impurity region 104 to between 5 to10 times the impurity concentration of the first impurity regions 103.Note that this impurity concentration is denoted as “n” (the “n” regionrefers to the second impurity regions 104 in this specification).

Further, each of the third impurity regions 105 has a length of between2 to 20 μm (typically between 3 to 10 μm), and includes a concentrationof a periodic table group 15 element in the range of 1×10¹⁹ to 1×10²¹atoms/cm³ (typically between 1×10²⁰ to 5×10²⁰ atoms/cm³). Each of thethird impurity regions 105 becomes a source region or a drain regionwhich provides an electrical connection between the source wiring, orthe drain wing, and the TFT. Note that this impurity concentration isdenoted as “n⁺” (the “n⁺” region refers to the second impurity regions105 in this specification).

In addition, in the present invention, each of the third impurityregions 10 plays a very important role for gettering, from the inside ofthe channel forming region 102, the catalytic element used incrystallization of the channel forming region. The effect thereof willnow be briefly explained.

In the present invention, a catalytic element for promoting thecrystallization (typically nickel) can be used during crystallization ofan amorphous semiconductor film. However, nickel is a metallic element,so it may be the cause of a leak current if any remains in the channelforming region. In other words, it is desirable to provide a process inwhich the catalytic element is at least removed from the channel formingregion after the catalytic element has been used.

The present invention is characterized in that a periodic table group 15element (preferably phosphorous) is used in the source region and thedrain region in order to remove the catalytic element. Namely, byconducting a heat treatment after forming the source region and thedrain region (the third impurity regions 105), the nickel that remainsinside the channel forming region 102 is gettered (captured) into thethird impurity regions 105. Thus, the catalytic element used forcrystallization can be removed from inside the channel forming region102.

Therefore, the gettered catalytic element collects in the third impurityregions 105, where it exists at high concentration. The presentapplicant investigated this by SIMS (secondary ion mass spectroscopy),and found that the concentration of the catalytic element to be between1×10¹⁷ to 1×10²⁰ atoms/cm³ (typically between 1×10¹⁸ to 5×10¹⁹atoms/cm³). However, the third impurity regions 105 may only perform thefunction as an electrode, so that the existence of a large amount of thecatalytic element will not generate any problems.

On the other hand, the concentration of the catalytic element in thechannel forming region 102 is greatly reduced (or eliminated) bygettering action. The present inventor investigated by SIMS and foundthat the concentration of the catalytic element had been reduced to2×10¹⁷ atoms/cm³ or less (typically, between 1×10¹⁴ to 5×10¹⁶ atoms/cm³)in the channel forming region. (Strictly speaking, a pad was formed tohave a composition identical to that of the channel forming region 102,and then measured by SIMS). Thus, a characteristic of the presentinvention resides in that there is a large difference in the catalyticelement concentration depending upon their positions (a difference of100 to 1,000 times) even within the same active layer.

The active layer of the NTFT according to the present invention, asdescribed above, is characterized in that it includes at least threeimpurity regions that include the same impurity with differentconcentrations other than the channel forming region. With employingsuch a structure, an arrangement can be realized in which the impurityconcentration (a periodic table group 15 element) gradually increases asthe distance from the channel forming region 102 to each of the firstimpurity regions 103, the second impurity regions 104, and the thirdimpurity regions 105 becomes long (in proportion to the distance fromthe channel forming region 102).

Further, since the object of the invention is to intentionally form aconcentration gradient like that seen in the LDD section of theconventional MOSFET example, by using a plurality of impurity regions,there is no problem if more than three impurity regions exist.

A gate insulating film 106 is formed on the active layer thus formed.The gate insulating film 106 is formed so that it overlaps the secondimpurity regions 104 for the case in FIG. 1. This is a unique processstructure when forming the second impurity regions 104, which becomes acharacteristic when the present invention is embodied. The gateinsulating film 106 is formed so as to be brought into contact with thechannel forming region 102, the first impurity regions 103, and thesecond impurity regions 104.

A gate wiring 107 is further formed on the gate insulating film 106. Asthe material for the gate wiring 107, a single metallic layer, an alloylayer, or a laminate structure of these combination may be employed suchas tantalum (Ta); tantalum nitrite (TaN); titanium (Ti); chromium (Cr);tungsten (W); molybdenum (Mo); silicon (Si); aluminum (Al); or Copper(Cu).

Typical examples of the laminate structure include the structurescomprising: Ta/Al; Ta/Ti; Cu/W; and Al/W. In addition, metallic silicidestructures (specifically, structures given conductivity with acombination of silicon and a metallic silicide such as Si/WSix,Si/TiSix, Si/CoSix, and Si/MoSix) may be employed.

However, when forming sidewalls made of silicon, it is preferred toplace a material on the top surface thereof with a high selectiveetching ratio compared to silicon. This is used to prevent etching allthe way to the gate wiring when forming the sidewalls. Otherwise, it isnecessary to use a protective film on the top surface as a stopper forprotection when forming the sidewalls.

Further, though it will be described later, a PTFT structure preparedwith no sidewall for the CMOS circuit of the present invention iseffective. Therefore, since a later step for removing only the sidewallsis include, it is necessary to choose a material for the gate wiringthat is not etched during removal of the sidewalls. In the paperdescribed as the conventional example, it has a structure with a silicongate directly contacting silicon sidewalls, it is not possible torealize the CMOS circuit of the present invention by using thatstructure as it is.

Further, when performing a heat treatment for the gettering processdescribed above, it is necessary to pay attention to the thermalresistance etc. of the gate wiring 107 (or a gate wiring 113). Arestriction on the temperature of the heat treatment arises if a lowmelting point metal such as aluminum is included. In addition, tantalumoxidizes very easily, and it is necessary to provide a protective filmof silicon nitride etc., so that the tantalum is not exposed to theenvironment of the heat treatment.

A silicon nitride film 108 shown in FIG. 1 is a protective film providedfor that reason. It is effective to dope a fine amount of boron into thesilicon nitride film 108, because this increases the thermalconductivity and can provide a radiating effect.

Sidewalls 109 are formed in the sidewalls (side portions) of the gatewiring 107. A layer with silicon as its main component is used as thesidewalls 109 for the present invention (specifically, a silicon layeror a silicon germanium layer). Especially, the use of intrinsic silicon,is desirable. Of course, an amorphous, crystalline, or microcrystallinestructure may be used.

The present invention takes a structure in which the sidewalls 109overlap on the first impurity regions 103 (the first impurity regions103 and the sidewalls 109 overlap through the gate insulating film 106).With this structure, advantages similar to the GOLD structure or theLATID structure of a MOSFET can be obtained.

Further, in order to realize this type of structure, it is necessary tohave a structure in which a voltage is applied to the first impurityregions 103 by the sidewalls 109. If the sidewalls 109 are formed ofintrinsic silicon layer, the resistance is high but a leak current isgenerated, so that there is a benefit in that the sidewall portion doesnot form a capacitor. Namely, it is possible to prevent a dielectricstorage capacity from being formed in the sidewalls when the gatevoltage is turned off.

In addition, the active layer film thickness becomes thin for the caseof a TFT, in the range of 20 to 50 nm, and during operation thedepletion layer expands completely to the lower part of the activelayer, and the TFT becomes a fully depression type (FD type). By makingthe FD type TFT into an overlapping gate type, an electric field isformed in the direction in which a hot carrier is difficult to generate.On the contrary, by using a general offset structure with an FD typeTFT, there is the fear that an electric field will be formed in adirection that promotes hot carrier injection.

The NTFT of the present invention can achieve a high reliability that isequal to, or greater than, that of a MOSFET by using a structure likethe one described above. Further, by applying a gate voltage to thefirst impurity regions 103 by using the sidewalls 109, it is possible toachieve an effect that is similar to that of an overlap structure.

Next, by arranging each of the first impurity regions 103, each of thesecond impurity regions 104, and each of the third impurity regions 105in the stated order, a structure can be realized in which the impurityconcentration gradually becomes higher from the channel forming region102 toward the source region (or drain region). With employing thisstructure, it is possible to effectively control the TFT off current.

In addition, the second impurity region 104 is set some distance apartfrom the gate voltage, so that an electric field relaxation effect,similar to that of the overlap portion of the MOSFET shown in FIG. 2A,can be obtained. Further, the hot carrier that is generated by the firstimpurity regions 103 is injected straight up toward the sidewalls 109,so a trap state is not formed directly above the channel forming region102.

The NTFT of the present invention is explained above, however ap-channel type TFT (hereinafter referred to as PTFT) is basically astructure in which an LDD region and an offset region are not provided.Of course, it may employs a structure in which an LDD region and anoffset region are arranged, however PTFTs have always had highreliability, so that it is desirable to gain the on current and balancethe characteristics with the NTFT. As shown in FIG. 1, this balance ofcharacteristics is especially important for applying the presentinvention to a CMOS circuit. However, it may apply the structure of thepresent invention to a PTFT.

The active layer of a PTFT in FIG. 1 possesses a channel forming region110 and a pair of fourth impurity regions 111, which become the sourceregion (or the drain region). Note that the impurity concentration (anelement from periodic table group 13, boron is typical) is denoted as“p⁺⁺” (the fourth impurity regions 111 are referred to as “p⁺⁺”throughout this specification).

The fourth impurity regions 111 invert to p-type due to the periodictable group 13 element, but if a periodic table group 15 element isdoped to the same concentration as in the third impurity regions 105 ata previous step, a sufficient gettering effect will be shown.

Therefore, catalytic element used for crystallization also exist in thefourth impurity regions 111 at a concentration of 1×10¹⁷ to 1×10²⁰atoms/cm³ (typically, 1×10¹⁸ to 5×10¹⁹ atoms/cm³) for this case. Sincethe fourth impurity regions 111 may only perform the function as anelectrode, there is no problem if the catalytic element is present inlarge amounts. Of course the catalytic element concentration in thechannel forming region 110 is 1/100 to 1/1000 times that in the fourthimpurity regions 111, so the concentration is 2×10¹⁷ atoms/cm³ or less(1×10¹⁴ to 5×10¹⁶ atoms/cm³ is typical).

In addition, a gate insulating film 112 is formed in a self-aligningmanner using the gate wiring 113 as a mask. As the characteristics ofthe process of the present invention, there are enumerated such factsthat the sidewalls 109 are present in the NTFT, and the sidewalls areremoved and do not remain in the PTFT.

The NTFT and PTFT formed in this manner are then covered by the firstinsulating films 114 (which also may be referred to as first interlayerinsulating films), and source wirings 115 and 116, and a drain wiring,117 are formed. After forming these wirings in the structure of FIG. 1,a silicon nitride layer 118 is formed as a protective film to therebyincrease the passivation effect. A second insulating layer 119 is formedout of a resin material on the silicon nitride layer 118. It is notnecessary to place restrictions to the use of a resin material, howeverusing a resin material is effective in maintaining flatness. Note thatfor a case in which another film is formed on the second insulating film119, it is acceptable to denote the second insulating film 119 by“second inter layer insulating film.”

A CMOS circuit in which an NTFT and a PTFT are combined in acomplimentary manner has been explained thus far, however it is possibleto apply the present invention to an NMOS circuit using an NTFT and to apixel TFT formed from NTFTs. Of course, it can also be applied to acomplex semiconductor circuit in which a CMOS circuit is taken as abasic unit.

In addition, the most characteristic point of the present inventionresides in that it is formed in several stages so that the impurityconcentration in the LDD region of an NTFT increases as a distance fromthe channel forming region increases. Furthermore, the catalytic element(an element used during crystallization) inside the channel formingregion is reduced to a level in which it does not hinder the electricalcharacteristics of the TFT.

Thus it is not necessary to place limits on the TFT structure providedthat this structure is included, and the present invention can beapplied to a top gate structure (a planer structure is typical) and to abottom gate structure (an inverted stagger structure is typical).

(Advantages of the NTFT Structure of the Present Invention)

The advantages of the NTFT structure of the present invention are nowdiscussed. The present invention's NTFT structure is characterized by amultiply-formed LDD region, from the first impurity regions 103 (the 1stLDD regions) and the second impurity regions 104 (the 2nd LDD regions),ones of which are overlapped by a gate electrode.

The superiority of the present invention will be explained by using acomparison with a conventional structure. FIGS. 19A and 19B show an NTFTwith no LDD structure and its electrical characteristics (characteristicgate voltage Vg vs. drain current Id), respectively. The same is shownin FIGS. 19C and 19D for the case of a normal LDD structure, in FIGS.19E and 19F for the so-called GOLD structure, and in FIGS. 19G and 19Hfor the NTFT of the present invention.

Note that throughout the figures, “n⁺” denotes the source region or thedrain region, channel denotes the channel forming region, and “n⁻”denotes the LDD region (“n” denotes the second LDD region). Further,“Id” is the drain current, and “Vg” is the gate voltage.

The off current is high if there is no LDD structure, as shown in FIGS.19A and 19B, and the on current (the drain current when the TFT is in anon state) and off current easily degrade.

Next, for the case with an LDD structure, the off current isconsiderably suppressed, and the degradation of the on current and theoff current can be suppressed. However, the degradation of the oncurrent cannot be completely suppressed.

Then, the structure in which the LDD region and the gate electrodeoverlap (FIGS. 19C and 19D) is a structure that places great importanceon suppressing the degradation of the on current found in a conventionalLDD structure.

While the on current degradation can be sufficiently suppressed for thiscase, it has the problem of the off current being somewhat higher thanin a normal LDD. This structure is employed by Hatano et al. in theirpaper which is the conventional example, but the high off currentproblem is recognized for the present invention, and a structure tosolve the problem is searched out.

For the structure of the present invention, as shown in FIGS. 19G and19H, the inner LDD region (the side closer to the channel formingregion) overlaps the gate electrode, while the outer LDD region isformed so as not to overlap the gate electrode. By employing thisstructure, it is possible to reduce the off current while maintainingthe effect that suppresses the on current degradation.

To the question of why does the off current get large in the structureshown in FIGS. 19E and 19F, the present inventor surmises the following.This explanation is made using FIGS. 20A and 20B.

When the NTFT is in an off state, a negative voltage of several dozenvolts is applied to a gate electrode 41. In this state, if a positivevoltage of several dozen volts is placed on a drain region 42, a verylarge electric field is formed at the edge portion on the drain side ofa gate insulating film 43.

At this time, holes 45 which are minority carriers are induced in an LDDregion 44, as shown in FIG. 20A. An energy band diagram is shown in FIG.20B for this time. Namely, the minority carriers form a current paththat connects the drain region 42, the LDD region 44, and a channelforming region 46. This current path is considered to bring about anincrease in the off current.

In order to interrupt this current path along the way, the presentapplicant considers that it is necessary to form a separate resistivemember in the location where the gate electrode is not overlapped, inother words a second LDD region. The structure of the present inventionhit upon in this way.

The structure of the present invention outlined above is explained indetail by the embodiments shown below.

Embodiment 1

In embodiment 1, the manufacturing method of the CMOS circuit shown inFIG. 1 is explained using FIGS. 3A to 3E, FIGS. 4A to 4D, and FIGS.5A-5B.

First, a 200 nm silicon oxide film 302 that becomes a base film isformed on a glass substrate 301. It may laminate a silicon nitride filmonto the base film, and may use only a silicon nitride film. Plasma CVD,thermal CVD, or sputtering may be used as a film deposition method. Ofcourse, doping boron into the silicon nitride film is effective toincrease the radiation effect.

Next, a 50 nm amorphous silicon film is formed by plasma CVD, thermalCVD, or sputtering, on the silicon oxide film 302. Crystallization ofthe amorphous silicon film is carried out afterward by using thetechnique disclosed in Japanese Patent Application Laid-open No. Hei7-130652, forming a semiconductor film containing crystals. This processis explained using FIGS. 5A and 5B.

First, a silicon oxide film 502 is formed as a base film on a glasssubstrate 501, and an amorphous silicon film 503 is formed thereon. Filmdeposition of the silicon oxide film 502 and the amorphous silicon film503 is performed successively by sputtering in embodiment 1. Next, a 10ppm nickel by weight of nickel acetate salt solution is applied, forminga nickel containing layer 504. (See FIG. 5A.)

Note that it is acceptable to use any of the following elements, eithersingly or in combination, instead of nickel (Ni); germanium (Ge); iron(Fe); palladium (Pd); tin (Sn); lead (Pb); cobalt (Co); platinum (Pt);copper (Cu); gold (Au); and silicon (Si).

Next, after a dehydrogenating process at 500° C. for 1 hour, a heattreatment is performed at a range between 500 and 650° C. for 4 to 24hours (at 550° C. for 14 hours in embodiment 1), forming a polysiliconfilm 505. The polysilicon film 505 formed in this way is known topossess outstanding crystalline characteristics. (See FIG. 5B.)

However, a high concentration of nickel used for crystallization existsinside the polysilicon film 505 at this point. It is experimentallyverified that a nickel with a concentration from 1×10¹⁸ to 1×10¹⁹atoms/cm³ at the minimum value of the measurement value by SIMS(secondary ion mass spectroscopy) remains. This nickel easily silicifiesinside the channel forming region, and there is a concern that it willfunction as a low resistance current path (leak current flow path).

Note that the present applicant has investigated the electriccharacteristics of an actual TFT and was able to confirm that a nickelconcentration at this level causes no remarkable bad influence on theelectrical characteristics of the TFT. However, so long as there is onlya chance of a bad influence, it is desirable to remove the nickel fromat least the channel forming region. A gettering process for thispurpose is explained below.

After forming the polysilicon film 505 in this manner, it is patternedinto an island shape and active layers 303 and 304 are formed as shownin FIG. 5A.

Note that it is acceptable to radiate the silicon film 505 with excimerlaser light after it is formed. This may further be performed afterforming the active layers 303 and 304. Any well-known method may be usedfor the excimer laser irradiation process, so its explanation is omittedhere.

Next, a gate insulating film 305 is formed from an silicon oxide nitridefilm (expressed by SiOxNy) over the active layers 303 and 304, and gatewirings (including gate electrodes) 306 and 307 with a laminatestructure of tantalum and tantalum nitride are formed thereon. (See FIG.3A.)

The gate insulating film 305 has a thickness of 120 nm. Of course, asilicon oxide film or a laminate structure of silicon oxide and siliconnitride films may be used instead of the silicon oxide nitride film. Inaddition, it is possible to use another metal for the gate wirings 306and 307, however a material with a high selective etching ratio tosilicon is desirable in view of later processes.

A first phosphorous doping process (a process of doping with phosphorus)is performed after obtaining the conditions of FIG. 3A. The accelerationvoltage is set high to 80 KeV because the doping must be performedthrough the gate insulating film 305 here. Further, the length (width)of first impurity regions 308 and 309 formed in this way is regulated to0.5 μm and the phosphorous concentration is regulated to 1×10¹⁷atoms/cm³. Note that arsenic may be used as a substitute forphosphorous.

Also, the first impurity regions 308 and 309 are formed by using thegate wirings 306 and 307 as masks in a self-aligning manner. At thispoint an intrinsic polysilicon layer remains directly under the gatewirings 306 and 307, where channel forming regions 310 and 311 areformed. However, in practice, some amount will wrap around the inside ofthe gate wiring and be doped, resulting in a structure in which the gatewirings 306 and 307, and the first impurity regions 308 and 309 overlap.(See FIG. 3B.)

Next, an amorphous silicon film with a thickness from 0.1 to 1 μm(Typically, 0.2 to 0.3 μm) is formed so as to cover the gate wirings 306and 307, and sidewalls 312 and 313 are formed by anisotropic etchingusing a chlorine gas. The width of the sidewalls 312 and 313 (thethickness viewed from the side portion of the gate wiring) is set as 0.2μm. (See FIG. 3C.)

Note that an amorphous silicon layer with no doped impurities is used inembodiment 1, so that the sidewalls are formed from an intrinsic siliconlayer (undoped silicon layer).

After the conditions in FIG. 3C are obtained, a second phosphorousdoping process is performed. The acceleration voltage is set at 80 KeVin this case, the same as for the first doping process. Further, thedose is regulated so that the phosphorous concentration is 1×10¹⁸atoms/cm³ in second impurity regions 314 and 315 formed at this time.

Note that the first impurity regions 308 and 309 remain only directlyunder the sidewalls 312 and 313 in the phosphorous doping process shownin FIG. 3D. Namely, this process defines the first impurity region 103shown in FIG. 1. The first impurity region 308 functions as the 1st LDDregion of the NTFT.

Phosphorous is also doped into the sidewalls 312 and 313 in the processof FIG. 3D. In practice, the acceleration voltage is high, so that thephosphorous distribution is known to be in a state where the tail end(hem) of the phosphorous concentration profile extends to the insideportion of the sidewalls 312 and 313. Although the resistive componentof the sidewalls can be regulated by this phosphorous, if thephosphorous concentration distribution shows extreme dispersion, thismay cause the gate voltage applied to the first impurity region 308 tofluctuate element by element. Therefore, precise control is necessarywhen doping.

Next, a resist mask 316 is formed covering a portion of the NTFT. Thesidewall 313 on the PTFT is first removed, and a part of the gateinsulating film 305 is then dry etched, forming processed gateinsulating films 317 and 318. (See FIG. 3E.)

At this point, the length of the portion of the gate insulating film 317projecting outwardly beyond the sidewall 312 (the length of the portionof gate insulating film 317 contacting a second impurity region 314)determines by the length (width) of the second impurity region 104 shownin FIG. 1. Conventionally, however, there has been a single LDD region,so dispersion in the width had a large influence on the electricalcharacteristics. Even if there is some dispersion in the width of thesecond impurity region 314, however, this will not become a problembecause there are substantially two kinds of LDD regions in the case ofembodiment 1.

On the other hand, the gate insulating film 318 is formed in aself-aligning manner using the gate wiring 307 as a mask. Consequently,the first impurity region 309 and the second impurity region 315 haveexposed shapes.

After the state in FIG. 3E is obtained, a third phosphorous dopingprocess is performed. The acceleration voltage is set low at 10 KeVbecause the exposed active layer is doped with phosphorous this time.Note that in embodiment 1, the dose is regulated so that a phosphorouswith a concentration of 5×10²⁰ atoms/cm³ is contained in third impurityregions 319 and 320. (See FIG. 4A.)

The portion shielded by the resist mask 316 (the NTFT side) in thisprocess is not doped with phosphorous, so that the second impurityregion 314 remains as is in that section. Namely, this process definesthe second impurity region 104 shown in FIG. 1, and at the same timedefines the third impurity region 105 shown in FIG. 1. The secondimpurity region 104 functions as the 2nd LDD region, and the thirdimpurity region 105 functions as the source region or the drain region.

In addition, since phosphorous is doped in the active layer that becomesthe PTFT with the gate wiring 307 as a mask, a third impurity region 320is formed in a self-aligning manner. At this point the phosphorous doseis 5 to 10 times higher than the above-mentioned second phosphorousdose, so the first impurity region (“n⁻” region) and the second impurityregion (“n” region) are essentially the same as the third impurityregion (“n⁺” region).

Note that it is desirable to regulate the phosphorous dopant amount sothat a concentration in the third impurity regions 319 and 320 is atleast 1×10¹⁹ atoms/cm³ or greater (preferably, between 1×10²⁰ and 5×10²¹atoms/cm³) in embodiment 1. With a concentration lower than this, thereis a fear that one cannot expect effective phosphorous gettering.

Next, the resist mask 316 is removed, and a resist mask 321 is newlyformed to cover the NTFT. Then a boron doping process (a process ofdoping with boron) is performed. The acceleration voltage is set to 10KeV here, and the dose is regulated so that a boron concentration of3×10²¹ atoms/cm³ is contained in a formed fourth impurity region 322.The boron concentration at this time is denoted as “p⁺⁺”. (See FIG. 4B.)

The third impurity region 320 (n⁺) formed on the PTFT side is invertedby boron in this process, becoming p-type. Therefore, phosphorous andboron are mixed in the fourth impurity region 322. Further, the portionformed which wraps around the inside of the gate wiring 307 is alsoinverted into p-type by boron wrap around.

This defines the fourth impurity region 111 shown in FIG. 1. The fourthimpurity region 322 is formed in a self-aligning manner using the gatewiring 307 as a mask, and functions as the source region or the drainregion. In embodiment 1, neither the LDD region nor the offset regionare formed for the PTFT, but there is no problem because PTFTsinherently have high reliability. On the contrary, one can gain the oncurrent by not forming an LDD region etc., so there are cases when it isconvenient not to do so.

This leads finally to an NTFT active layer in which a channel formingregion, a first impurity region, a second impurity region, and a thirdimpurity region are formed, and to a PTFT active layer in which achannel forming region and a fourth impurity region are formed, as shownin FIG. 4B.

The resist mask 321 is removed after obtaining the conditions of FIG.4B, and then a silicon nitride film 323 is formed as a protective film.The silicon nitride film 323 has a film thickness of from 1 to 100 nm(typically from 5 to 50 nm, preferably, from 10 to 30 nm).

Next, an annealing process is carried out at a processing temperaturebetween 500 and 650° C. (typically between 550 and 600° C.) for 2 to 24hours (typically from 4 to 12 hours). The annealing process is performedin a nitrogen atmosphere at 600° C. for 12 hours in embodiment 1. (SeeFIG. 4C.)

The annealing process is performed with the objective of activating thedoped impurities (phosphorous and boron) in the first impurity region308, the second impurity region 314, the third impurity region 319, andthe fourth impurity region 322, and at the same time gettering thenickel remaining, in the channel forming regions 310 and 311.

The phosphorous doped into the third impurity region 319 and the fourthimpurity region 322 getters the nickel in the annealing process. Namely,the nickel is captured by migrating in the direction of the arrow andcombining with phosphorous. Thus, nickel gathers in high concentrationin a third impurity region 324 and a fourth impurity region 325 shown inFIG. 4C. Specifically, nickel is present in both impurity regions at aconcentration of from 1×10¹⁷ to 1×10²⁰ atoms/cm³ (typically from 1×10¹⁸to 5×10¹⁹ atoms/cm³). At the same time, the nickel concentration withinthe channel regions 310 and 311 falls to 2×10¹⁷ atoms/cm³ or less(typically, from 1×10¹⁴ to 5×10¹⁶ atoms/cm³).

At this point, the silicon nitride film 323 formed as a protective filmprevents a tantalum film, used as a gate wiring material, fromoxidizing. There is no problem if the gate wiring is difficult tooxidize, or if the oxide film formed by oxidation is easily etched, butnot only is the tantalum film easily oxidized, an oxidized tantalum filmis extremely difficult to etch. Therefore, it is desirable to form theprotective silicon nitride film 323.

A first insulating film 326 is formed with a thickness of 1 m after thecompletion of the annealing process (gettering process) shown in FIG.4C. A silicon oxide film, a silicon nitride film, an oxide siliconnitride film, an organic resin film, or a laminate of these films can beused as the first insulating film 326. An acrylic resin film is employedin embodiment 1.

After formation of the first insulating film 326, source wirings 327 and328, and a drain wiring 329 are formed from a metallic material. Alaminate wiring structure, in which a titanium-containing aluminum filmis sandwiched with titanium, is used in embodiment 1.

Further, if BCB (benzocyclobutane) resin film is used as the firstinsulating film 326, the flatness will increase and at the same time itwill become possible to use copper as a wiring material. The wiringresistance is low with copper, so it is an extremely effective wiringmaterial.

After forming the source wiring and the drain wiring in this manner, asilicon nitride film 330 is formed with a thickness of 50 nm as apassivation layer. In addition, a second insulating film 331 is formedthereon as a protective film. The same material used for the firstinsulating film 326 may be used for the second insulating film 331. Alaminate structure with an acrylic resin film on a 50 nm thick siliconoxide film is employed in embodiment 1.

A CMOS circuit with the structure shown in FIG. 4D is completed throughthe above processes. For the CMOS circuit formed in embodiment 1, thereliability of the entire circuit sharply improves because the NTFT hasan excellent reliability. Further, the characteristic balance (balanceof electric characteristics) between the NTFT and the PTFT improves witha structure like that of embodiment 1, so making it more difficult for adefective operation to occur.

Further, the influence of nickel (catalyst element) inside the channelforming region, which is concerned about when the technique disclosed inthe Japanese Patent Application Laid-open No. Hei 7-130652 as a priorart is employed, is resolved by performing a gettering process as inembodiment 1.

Note that the structure described in embodiment 1 is merely one example,and therefore has no necessary to limit embodiment 1 to the structuresshown in FIGS. 3A to 3E, and in FIGS. 4A to 4D. The essential point inthis invention is the structure of the NTFT active layer, and as long asthat point is not changed, the effect of the present invention can beobtained.

Embodiment 2

Undoped Si (an intrinsic silicon layer or an undoped silicon layer), inwhich impurities are intentionally not doped, is used in embodiment 1for the sidewalls. However, in embodiment 2, a phosphorous doped siliconlayer (“n⁺”-Si layer), in which phosphorous is doped during filmformation, or a boron doped silicon layer (“p⁺”-Si layer) is used. It isneedless to say that the silicon layer may be amorphous,non-crystalline, or micro-crystalline.

By using a silicon layer doped with phosphorous or boron, the entiresidewall portion is made low resistance, and the possible change incharacterstics originating in fluctuation of the phosphorousconcentration profile can be eliminated, which is the fear of processshown in FIG. 3D.

Embodiment 3

Undoped Si (an intrinsic silicon layer or an undoped silicon layer), inwhich impurities are intentionally not doped, is used in embodiment 1for the sidewalls. However, in embodiment 3, a silicon layer thatcontains either of carbon (C), nitrogen (N), or oxygen (O) is used, tothereby increase the resistive component of the sidewalls. Of course,the silicon layer may be either amorphous, crystalline, ormicro-crystalline. Further, oxygen is best as the impurity to be used.

Namely, it may only dope with carbon, nitrogen, or oxygen at 1 to 50atomic % (typically, from 10 to 30 atomic %) when forming the siliconlayer that becomes the sidewalls. Oxygen at 20 atomic % is doped in thisembodiment 3.

With employing the structure of embodiment 3, the resistive componentdue to the sidewalls becomes larger. Accordingly, it may take such astructure that the capacitive component in which the sidewalls act asthe dielectric in response to the applied gate voltage becomesmanagingly effective. Namely, when driven at high frequency, the gatevoltage can also be effectively applied to the sidewall portion.

Embodiment 4

A description will be made of an example in the case where asemiconductor film including the crystals that become the active layerin embodiment 1 is crystallized by using the technique disclosed inJapanese Patent Application Laid-pen No. Hei 8-78329. Note that thetechnique disclosed in Japanese Patent Application Laid-open No. Hei8-78329 involves one being able to selectively crystallize asemiconductor film by selectively doping a catalytic element. FIGS. 6Aand 6B explain the case where this technique is applied to the presentinvention.

First, a silicon oxide film 602 is formed on a stainless steel substrate601, and an amorphous silicon film 603 and a silicon oxide film 604 areformed successively thereon. The silicon oxide film 604 has a thicknessof 150 nm at this time.

Next, the silicon oxide film 604 is patterned to form an opening portion605, and thereafter a 100 ppm by weight of nickel acetate salt solutioncontaining nickel is coated. This becomes a state in which a formednickel containing layer 606 only contacts the amorphous silicon tarn 602through the bottom portion of the opening portion 605. (See FIG. 6A.)

Next, crystallization of the amorphous silicon film 602 is performed bya heat treatment at between 500 and 650° C., for 4 to 24 hours (at 580°C. for 14 hours in this embodiment 4). First, the portion in contactwith nickel crystallizes, then the crystal growth proceeds almostparallel to the substrate during this crystallization process. Incrystallography terms, it has been confirmed that crystallizationproceeds in the <111> axis direction.

A polysilicon film 607 formed in this way is a collection of bar-like orneedle-like crystals, and has the advantage of matching crystallinitysince each of the cylindrical crystals grows in a specific directionmacroscopically.

Note that the following elements can be used instead of nickel (Ni) forthe above published technique, either singly or in combination:germanium (Ge); iron (Fe); palladium (Pd); tin (Sn); lead (Pb); cobalt(Co); platinum (Pt); copper (Cu); gold (Au); and silicon (Si).

A semiconductor film (including polysilicon films and polysilicongermanium films) containing crystals may be formed using the abovetechnique, which may then undergo patterning to form a semiconductorfilm active layer containing crystals. Further processing may beperformed in accordance with embodiment 1. It is needless to say that itis possible to combine embodiments 2 and 3 as well.

When manufacturing a TFT by using a semiconductor film containingcrystals that have been crystallized using the technique of thisembodiment 4, it is possible to obtain high electric field effectmobility. However, a high reliability is demanded thereto. However, byemploying the TFT structure of the present invention, it is possible tomanufacture a TFT that maximizes the technique of this embodiment 4.

Embodiment 5

In this Embodiment 5, a description is made of an example in whichembodiment 1 is combined with the techniques disclosed in JapanesePatent Application Laid-open No. Hei 10-135468 or Japanese PatentApplication Laid-open No. Hei 10-135469.

The techniques described in these patent publications involve using thegettering action of a halogen element, after crystallization, to removethe nickel that has been used during crystallization of thesemiconductor. The nickel concentration in the active layer can bereduced to 1×10¹⁷ atoms/cm³ or less (preferably, 1×10¹⁶ atoms/cm³ orless) by using these techniques.

FIGS. 7A and 7B are used to explain the structure of embodiment 5.First, a quartz substrate 701 with high heat resistance is used. Ofcourse, silicon substrates and ceramic substrates may also be used. Whena quartz substrate is used, there is no contamination from the substrateside even if a silicon oxide film is not particularly formed as a basefilm.

Next a polysilicon film (not shown) is formed using the methodsdescribed in embodiment 1 or embodiment 4, then patterned to form activelayers 702 and 703. In addition, a gate insulating film 704 is formedfrom a silicon oxide film, covering the active layers 702 and 703. (SeeFIG. 7A.)

A heat treatment is performed in an atmosphere containing halogen afterthe gate insulating film 704 is formed. The processing atmosphere ofthis embodiment 5 is an oxidizing atmosphere combining oxygen andhydrogen chloride, the processing temperature is 950° C., and theprocessing time is 30 minutes. Note that the processing temperature maybe set from 700 to 1,150° C. (typically, from 800 to 1,000° C.), and theprocessing time may be set from 10 minutes to 8 hours (typically from 30minutes to 2 hours). (See FIG. 7B.)

At this time, the nickel becomes a volatile nickel chloride compound andspreads throughout the processing atmosphere, and the nickelconcentration in the polysilicon film is reduced. Therefore, theconcentration of nickel in the active layers 705 and 706 shown in FIG.7B is reduced to 1×10¹⁷ atoms/cm³ or less.

After forming an active layer using the techniques of embodiment 5above, further processing may be performed in accordance withembodiment 1. Of course, it is also possible to combine with any ofembodiments 2 to 5. It has especially been shown that an extremely highcrystallinity in the polysilicon film can be achieved with a combinationof embodiment 5 with embodiment 4.

(Knowledge Related to the Crystal Structure of the Active Layer)

Looking microscopically at a semiconductor layer (active layer) formedin accordance with the above manufacturing processes, one finds acrystal structure consisting of multiple needle-like or bar-likecrystals (hereafter referred to as bar-like crystals). It is easy toconfirm this by observation using a TEM (transmission electronmicroscope).

In addition, it has been verified by using electron diffraction andx-ray diffraction that although there is some crystal axis deviation onthe surface of the active layer (the channel forming portion), theprincipal orientation face is {110}. As a result of detailed observationof electron beam diffraction photographs with a spot diameter of 1.5 μm,the present applicant found that the diffraction spot appeared cleanlyin correspondent with the {110} face, and that each spot had aconcentric distribution.

Further, the present applicant observed the grain boundaries formed byeach of the contacting bar-like crystals using an HR-TEM (highresolution transmission electron microscope) and verified that thecrystal lattice in the grain boundaries has continuity. This was easilyverified by the continuous connection of the observed lattice stripes inthe grain boundaries.

Note that the continuity of the lattice in the grain boundariesoriginates in the fact that the grain boundaries are “planar shape grainboundaries.” The definition of planar shape boundaries in thisspecification is described in “characterization of High-EfficiencyCast-Si Solar Cell Wafers by MBIC Measurement; Ryuichi Shimokawa andYutaka Hayashi, Japanese Journal of Applied Physics vol. 27, no. 5, pp.751-758, 1988.”

According to the above paper, planar shape grain boundaries includestwin crystal grain boundaries, special stacking faults, special twistgrain boundaries, etc. This planar shape grain boundary possesses acharacteristic in that it is inactive electrically. Namely, the grainboundaries can essentially be seen as non-existent, because they do notfunction as a trap that obstructs the movement of a carrier.

Especially for the cases where the crystal axis (the axis perpendicularto the crystal face) is the <110> axis, {211} twin crystal grainboundaries can be called grain boundaries corresponding to Σ3. The Σvalue is a parameter that indicates the degree of matching incorresponding grain boundaries, and it is known that the smaller Σ valuethe better matching grain boundary is.

Using a TEM, the present applicant observed in detail a polysilicon filmobtained by implementing the present invention, and determined that mostof the crystal grain boundaries (90% or more, typically 95% or more) hadgrain boundaries corresponding to Σ3. In other words, the grainboundaries were {211} twin grain boundaries.

For the case of two crystals having a {110} face orientation, if thelattice stripes corresponding to the {111} face in the grain boundaryformed between both crystals has an angle θ, then when θ=70.5°, thegrain boundaries correspond to Σ3.

Neighboring grain lattice stripes in the grain boundaries of thepolysilicon film in this embodiment 5 is continuous at just about 70.5°.From this one can arrive at the conclusion that the grain boundaries are{211} twin grain boundaries.

Note that θ=38.9° corresponds to Σ9, aid other grain boundaries likethis also exist.

This type of grain boundary correspondence is only formed between grainsin the same face orientation. In other words, the polysilicon filmobtained in this embodiment 5 has a face orientation roughly matched to{110}, and therefore this grain boundary correspondence is formed over awide area.

This type of crystal structure (literally, grain boundary structure)shows that two different grains are joined together with extremely goodmatching in the grain boundaries. Namely, a crystal structure in whichthe crystal lattice has continuity in the grain boundaries, and in whichit is very difficult to create a trap caused by crystal defects etc.Therefore, it is possible to regard semiconductor thin films having thistype of crystal structure as ones in which grain boundaries do notsubstantially exist.

In addition, it has been verified by TEM that defects within the grainboundaries almost completely disappear with a heat treatment process ata high temperature of 700 to 1,150° C. It is evident that there is alarge decrease in the number of defects around this type of heattreatment process.

The difference in the number of defects appears as the difference inspin density by electron spin resonance (ESR). At present, polysiliconfilms manufactured in accordance with the processes of this embodiment 5have been shown to have a spin density of at most 5×10¹⁷ spins/cm³ orless (preferably, 3×10¹⁷ spins/cm³ or less). However, this measurementvalue is near the detection limits of the present measuring equipment,and it is expected that the real spin density is even lower.

From the above, the polysilicon film obtained by embodying thisembodiment 5 has essentially no internal grains or grain boundaries, sothat it can be thought of as a single crystal silicon film oressentially a single crystal silicon film. The present applicant calls apolysilicon film with this type of crystal, structure CGS (continuousgrain silicon).

Information regarding CGS is on record and can be referred to inJapanese Patent Application Laid-open No. Hei 10-044659, Japanese PatentApplication Laid-open No. Hei 10-152316, Japanese Patent ApplicationLaid-open No. Hei 10-152308, and Japanese Patent Application Laid-openNo. Hei 10-152305 submitted by the applicant of the present invention.

(Knowledge Related to TFT Electrical Characteristics)

The TFT manufactured in this embodiment 5 showed electricalcharacteristics equivalent to a MOSFET. Data showing the following wasobtained from a TFT test manufactured by the present applicant.

(1) The subthreshold coefficient, which is an index of the switchingperformance (the quickness of on/off switching), is small at between 60and 100 mV/decade (typically from 60 to 85 mV/decade) for both ann-channel type TFT and a p-channel type TFT.

(2) The electric field effect mobility (μ_(FE)), which is an index ofthe TFT operation speed, is large at between 200 and 650 cm²/Vs(typically, between 300 and 500 cm²/Vs) for an n-channel type TFT, andbetween 100 and 300 cm²/Vs (typically between 150 and 200 cm²/Vs) for ap-channel type TFT.

(3) The threshold voltage (V_(th)), which is an index of the drivingvoltage for the TFT, is small at between −0.5 and 1.5 V for an n-channeltype TFT, and between −1.5 and 0.5 V for a p-channel type TFT.

The above verifies that it is possible to realize extremely excellentsuperior switching characteristics and high speed operation.

(Knowledge Related to Circuit Characteristics)

Next, the frequency characteristics of a ring oscillator manufacturedusing a TFT formed in accordance with this embodiment 5 are shown. Aring oscillator is a circuit in which CMOS structure inverter circuitsare connected in a ring state with an odd number of layers, and is usedto get a delay time in each of the inverter circuit layers. Thestructure of the oscillator used for the experiment is as follows.

Number of layers: 9 Film thickness of gate insulating film in TFT: 30 nmand 50 nm TFT gate length: 0.6 μm

The oscillation frequency of the ring oscillator was investigated, andthe largest oscillation frequency able to be obtained was 1.04 GHz.Further, one actual LSI circuit TEG, a shift register, was manufacturedand its operating frequency was verified. As a result, a 100 MHZ outputpulse operating frequency was obtained with a gate insulating film witha film thickness of 30 nm, a gate length of 0.6 m, a voltage supply of 5V, and 50 stages of a shift register circuit.

The amazing data above for a ring oscillator and a shift register showsthat the TFT of this embodiment 5 has a performance (electricalcharacteristics) equivalent to, or surpassing, a MOSFET.

Embodiment 6

Embodiment 6 shows an example in which, after forming a polysilicon filmusing a catalytic element (nickel is exemplified) as in embodiment 1 andembodiment 4, a process is performed to remove the nickel remainingthroughout the film. The techniques disclosed in Japanese PatentApplication Laid-open No. Hei 10-270363 and Japanese Patent ApplicationLaid-open 10-24775 are used as the nickel removing technique in thisembodiment 6.

The technique disclosed in Japanese Patent Application Laid-open No. Hei10-270363 is a technique that employs a periodic table group 15 element(phosphorous is typical) for a gettering action after thecrystallization process to remove the nickel that has been used duringthe semiconductor crystallization. Employment of this technique enablesto reduce the nickel concentration in the active layer to 1×10¹⁷atoms/cm³ or less (preferably, 1×10¹⁶ atoms/cm³ or less).

FIGS. 22A and 22B show a case where this technique is applied to thepresent invention. First, the processes of embodiment 1 shown in FIGS.5A and 5B are performed to form the polysilicon film 505. Next, aninsulating mask film 421 with an opening portion is formed, andphosphorous is doped in this state. At this time, a high concentrationof phosphorous is doped into the region of the polysilicon film exposedby the opening portion. The region formed is called a gettering region422. (See FIG. 22A)

The gettering region 422 is doped to a phosphorous concentration ofbetween 1×10¹⁹ and 1×10²¹ atoms/cm³ (typically, between 1×10¹⁷ and1×10²⁰ atoms/cm³).

Next, a heat treatments is performed at between 550 to 650° C. for 4 to15 hours (at 600° C. for 12 hours in this embodiment 6). The catalyticelement (nickel in this embodiment 6) remaining throughout thepolysilicon film 505 moves in the direction of the arrow during the heattreatment, and is captured (gettered) throughout the gettering region422. This region is labeled the gettering region 422 for this region. Apolysilicon film 423 formed in this way has a nickel concentrationthroughout the film that has been reduced to 1×10¹⁷ atoms/cm³ or less.

Furthermore, the technique disclosed in Japanese Patent ApplicationLaid-open No. Hei 10-247735 is a technique characterized in that afterperforming crystallization using the technique disclosed in JapanesePatent Application Laid-open No. Hei 8-78329, the mask used toselectively dope the catalytic element is left in place and used as amask for phosphorous dope. This technique is extremely effective forincreasing throughput.

A semiconductor film (including polysilicon films and polysilicongermanium films) containing crystals can be formed by using the abovetechniques according to this embodiment 6, patterned, and formed into anactive layer. Further processes may be carried out in accordance withembodiment 1. In addition, by combining embodiment 6 with the getteringtechnique of embodiment 1, it is possible to additionally reduce theamount of catalyst remaining in the channel forming region. It isneedless to say that it is also possible to combine this embodiment 6with any of embodiments 2 to 4.

Embodiment 7

FIGS. 8A to 8D show an example of embodiment 7, in which the formationprocesses for the silicon nitride film 323, formed by the getteringprocess (FIG. 4C) shown in embodiment 1, are performed at a differentstage.

First, processing is performed in accordance with embodiment 1 until theprocess of FIG. 3B, and then a 1-10 nm (preferably, from 2 to 5 nm)thick silicon nitride film 801 is formed. If the film thickness of thesilicon nitride film is too thick, however, the gate overlap structureused for a sidewall 802 can become impossible to realize, so that it ispreferable to make it thin. However, it is necessary to be careful notto harm the effect in which the gate wiring (for the case of tantalum)is protected from oxidizing during later heat treatment processes.

Next, an amorphous silicon film (not shown) is formed on the siliconnitride film 801, and sidewalls 802 and 803 are formed by anisotropicetching. (See FIG. 8A.)

Note that it is also possible to use a structure as in embodiment 2 orembodiment 3 for the structure of the sidewalls 802 and 803.

Next, a phosphorous doping process is performed in the state of FIG. 8A,to thereby form second impurity regions 804 and 805. Note thatconditions that are almost the same as those in embodiment 1 may beused; however, it is preferable to optimize the acceleration voltage andthe power with taking the film thickness of the silicon nitride film 801into consideration.

After forming the second impurity regions 804 and 805, resist masks 806and 807 are formed, and gate insulating films 808 and 809 are formed bydry etching of a part of the gate insulating film. (See FIG. 8B.)

Next, a phosphorous doping process is again performed in the state ofFIG. 8B, thereby forming a third impurity region 810. Then, afterremoving the resist masks 806 and 807, a resist mask 811 is formed andthe sidewall 803 is removed. A boron doping process is performed at thisstate. Note that conditions that are almost the same as those inembodiment 1 may be used, but as above, it is desirable to optimize theacceleration voltage and the power with taking the film thickness of thesilicon nitride film 801 into consideration. This forms a fourthimpurity region 812.

Note that the structure explained in embodiment 1 may be used regardingthe phosphorous concentration and the boron concentration contained inthe third impurity region 810 and the fourth impurity region 812. Ofcourse, there is no need to limit these to the values of embodiment 1.

After obtaining the conditions in FIG. 8C, a heat treatment process isperformed for gettering, at the same conditions as in embodiment 1.After the heat treatment process, a nickel concentration of between1×10¹⁷ and 1×10²⁰ (typically, between 1×10¹⁸ and 5×10¹⁹ atoms/cm³)remains in the third impurity region 813 and the fourth impurity region814. The relationship of the nickel concentration to the channel formingregion has already been explained.

A CMOS circuit is completed by performing, in order, the same processesas those of embodiment 1, after the above processes are finished. Thedifference between the structure of embodiment 7 and the structure shownin FIG. 1 is that the gate insulating film 809 exists on the PTFT sidefor the case of embodiment 7.

The structure and processes of this embodiment 7 do not impede theeffect of the present invention, and a semiconductor device with highreliability can be manufactured. Note that embodiment 7 can be freelycombined with any of embodiments 2 to 6.

Embodiment 8

In this embodiment 8, a description will be made of an example or a casein which the structure of embodiment 7 is varied, with reference toFIGS. 9A to 9D. Specifically, it is characterized in that thisembodiment 7 includes a process in which a silicon nitride film formedin order to protect the gate wiring is etched using the sidewalls asmasks.

First, processing is performed in accordance with the processes ofembodiment 1 until the processes of FIG. 8A. Thereafter, the sidewalls802 and 803 are used as a mask and the silicon nitride film 801 isetched, to thereby form silicon nitride films 901 and 902 with the shapeshown. (See FIG. 9A.)

Next, a phosphorous doping process is performed in the state of FIG. 9A,thereby forming second impurity regions 903 and 904. Note thatconditions that are almost the same as those in embodiment 1 may beused; however, it is desirable to optimize the acceleration voltage andthe power with taking the film thickness of the silicon nitride film 901into consideration.

After forming the second impurity regions 903 and 904, resist masks 905and 906 are formed, and gate insulating films 907 and 908 are formed bydry etching the gate insulating film. (See FIG. 9B)

Next, a phosphorous doping process is again performed in the state ofFIG. 9B to form a third impurity region 909. Then, after removing theresist masks 905 and 906, a resist mask 910 is formed and the sidewall803 is removed. A boron doping process is performed at this state. Notethat conditions of the boron addition process which are almost the sameas those in embodiment 1 may be used, but as above, it is desirable tooptimize the acceleration voltage and the power with taking the filmthickness of the silicon nitride film 901 into consideration. Thus, afourth impurity region 911 is formed.

Note that the structure explained in embodiment 1 may be used regardingthe phosphorous concentration and the boron concentration contained inthe third impurity region 909 and the fourth impurity region 911. Ofcourse, there is no need to limit these to the values of embodiment 1.

After the conditions in FIG. 9C is thus obtained, a heat treatment isperformed for gettering, at the same conditions as in embodiment 1.After the heat treatment process, a nickel concentration of between1×10¹⁷ and 1×10²⁰ (typically, between 1×10¹⁸ and 5×10¹⁹ atoms/cm³)remains in the third impurity region 912 and the fourth impurity region913. The relationship of the nickel concentration to the channel formingregion has already been explained.

A CMOS circuit is completed by performing, in order, the same processesas those of embodiment 1, after the above processes are finished. Thedifference between the structure of embodiment 8 and the structure shownin FIG. 1 resides in that the gate insulating film 902 and the siliconnitride film 908 exist on the PTFT side for the case of embodiment 8.

The structure and processes of this embodiment 8 do not impede theeffect of the present invention, and a semiconductor device with highreliability can be manufactured. Note that embodiment 8 can be freelycombined with any of embodiments 2 to 6.

Embodiment 9

An etching process of the gate insulating film 305 in FIG. 3E isperformed in embodiment 1, but that process can be omitted and the gateinsulating film 305 can remain until the final processing. A descriptionwill be made of this embodiment 8 by referring FIGS. 10A and 10B.

FIG. 10A shows the state of the gate insulating film 305 just beforeetching in FIG. 3E of embodiment 1. The processes of FIG. 4A to 4C areperformed in this state. At this time, the process phosphorous dopingprocess) shown in FIG. 4A becomes a through doping process (an impuritydoping process that goes through the insulating film). Therefore it isnecessary to set the acceleration voltage higher, at between 80 and 100KeV.

Additionally, the boron doping process of FIG. 4B similarly becomes athrough doping process. It is also necessary to set the accelerationvoltage higher in this case, at between 70 and 90 KeV.

Further, a CMOS circuit with the structure shown in FIG. 10B can beobtained by continuing to perform processing until the heat treatmentfor gettering. Note that this is structurally almost identical to thestructure shown in FIG. 1, so that a detailed explanation thereof isomitted here. Only numerals necessary for explaining its unique pointsare added here.

The structure of embodiment 9 is a state in which a third impurityregion 11 and a fourth impurity region 12 are completely covered by thegate insulating film 305. Namely, there is no worry of contamination tothe active layer from the processing environment because it is notexposed after the gate insulating film 305 is formed.

Further, a silicon nitride film 13, formed with the purpose ofprotecting the gate wiring, is formed with a shape that covers the gateinsulating film 305, the sidewall 312, and the gate wiring, and differsin this point from FIG. 1.

Note that embodiment 9 can be freely combined with the structure of anyof the embodiments 2 to 6.

Embodiment 10

Embodiment 10 is explained with reference to FIGS. 11A and 11B, in whichthe third impurity region on the NTFT side is formed by a bare dopingprocess (a process in which an impurity is directly doped into theactive layer, not going through an insulating film), and on the PTFTside is formed by a through doping process.

In embodiment 10, a resist mask 21 is formed at the same time as theresist mask 316 in FIG. 3E is formed. The gate insulating film 305 isetched using the resist mash 316 and 21 as masks, forming gateinsulating films 22 and 23. (See FIG. 11A)

In this state the processes until FIG. 4A to 4C are performed. Theprocess shown in FIG. 4A (a phosphorous doping process) is a bare dopingprocess, so the conditions at this time may be the same as those inembodiment 1. However, the boron doping process in FIG. 4B becomes athrough doping process, so it is necessary to set the accelerationvoltage high (between 70 and 90 KeV).

Further, a CMOS circuit with the structure shown in FIG. 11B can beobtained by continuing to perform processing until the heat treatmentprocess for gettering. Note that this is structurally almost identicalto the structure shown in FIG. 1, so a detailed explanation is omittedhere. Only numerals necessary for explaining its unique points are addedhere.

The structure of embodiment 10 is a state in which a third impurityregion 24 is not covered by the gate insulating film 22 (actually asmall amount of phosphorous wraps around, so that they overlap eachother), and a fourth impurity region 25 is completely covered by thegate insulating film 23.

Further, a silicon nitride film 26, formed with the purpose ofprotecting the gate wiring, is formed with a shape that covers the gateinsulating film 22, the third impurity region 24, the sidewall 312, andthe gate wiring, and differs in this point from FIG. 1.

Note that embodiment 10 can be freely combined with the structure of anyof the embodiments 2 to 6.

Embodiment 11

In embodiment 10, the third impurity region on the NTFT is formed by abare doping process, and the fourth impurity region on the PTFT isformed by a through doping process. In embodiment 11, the processes arereversed. Embodiment 11 is an example in which the third impurity regionon the NTFT is formed by a through doping process, and the fourthimpurity region on the PTFT is formed by a bare doping process.

In implementing embodiment 11, after the second phosphorous dopingprocess on the state in FIG. 11A, a new resist mask is formed tocompletely cover the NTFT, and the gate insulating film 305 is onlyetched on the PTFT side.

Doing this leads to a state in which only the NTFT active layer iscovered with the gate insulating film, and on the PTFT side the gateinsulating film remains only directly beneath the gate wiring. Furtherprocesses may be performed in accordance with embodiment 1 and theirexplanation is omitted here. However, the phosphorous doping processthat forms the third impurity region is replaced with a through dopingprocess, so it is necessary to set the acceleration voltage toapproximately 90 KeV (for this process).

Note that embodiment 11 can be freely combined with the structure of anyof embodiments 2 to 6.

Embodiment 12

Embodiment 1 was explained using a CMOS circuit as an example, but inembodiment 12, a case where the present invention is applied to a pixelmatrix circuit (display section) in an active matrix type liquid crystaldisplay panel is explained. FIGS. 15A to 15C are used for theexplanation. Note that FIG. 15B is across sectional structure diagramtaken along the A-A of FIG. 15A, and FIG. 15C corresponds to theequivalent circuit. Further, the pixel TFT shown in FIG. 15B is a doublegate structure in which the same structure NTFT is connected in series,so only one of them which is given by numerals is explained.

First, the following are all formed on a substrate 1500 in accordancewith the processes of embodiment 1: a base film 1501; a channel formingregion 1502; a first impurity region 1503; a second impurity region1504; third impurity regions 1505 and 1506; a gate insulating film 1507;a gate wiring 1509; a sidewall 1508; a silicon nitride film 1510; afirst insulating film 1511; a source wiring 1512; and a drain wring1513.

Then a silicon nitride film 1514 and a second insulating film 1515 areformed as passivation films on each of the wirings. In addition, a thirdinterlayer insulating film 1516 is formed thereon, and a pixel electrode1518 is formed out of a transparent conductor film such as ITO (indiumtin oxide), SnO₂, or a zinc oxide/indium oxide compound. Referencenumeral 1517 also denotes a pixel electrode.

In addition, the capacitive section is formed by: a capacitive wiring1522 as the upper portion electrode; an undoped silicon layer 1519 (anintrinsic semiconductor layer or a semiconductor layer doped with boronto a concentration between 1×10¹⁶ and 5×10¹⁸ atoms/cm³) together with animpurity region 1520 (containing phosphorous at the same concentrationas the that of the first impurity region 1503) as the lower portionelectrode; and an insulating film 1521 (formed at the same time as thegate insulating film 1507) sandwiched by the upper and lower electrodes.Note that the capacitive wiring 1522 is formed at the same time as thegate wiring 1509 on the pixel TFT, and is electrically connected toeither ground or to a fixed voltage source.

Further, the insulating film 1521 has the same material composition asthe gate insulating film 1507 on the pixel TFT, and the undoped siliconlayer 1519 has the same material composition as the channel formingregion 1502 on the pixel TFT.

Therefore, integration is possible by manufacturing the pixel TFT, thecapacitive section, and the CMOS circuit at the same time on the samesubstrate. A transmission type LCD is taken as one example and explainedfor embodiment 12, but of course there is no need to limit embodiment 12to that.

For example, it is possible to manufacture a reflective type LCD byusing a reflective conducting material for the pixel electrode, andappropriately changing the patterning of the pixel electrode or eitheradding or reducing processes.

In addition, a double gate structure is used for the gate wiring on thepixel TFT of the pixel matrix circuit in embodiment 12, but in order toreduce fluctuations in the off current, a triple gate structure or othermultiple gate structure may be employed. Further, a single gatestructure may be used in order to increase the opening ratio (degree).

Note that the structure of embodiment 12 can be freely combined with thestructure of any of embodiments 1 to 11.

Embodiment 13

An example of embodiment 13, in which the capacitive section having adifferent structure from that of embodiment 12 is formed with adifferent structure, is shown in FIG. 16. The basic structure is nearlythe same as that of embodiment 12, so only the points of difference willbe explained. The capacitive section of embodiment 13 is formed by animpurity region 1602 (containing the same phosphorous concentration asthat of the second impurity region) connected to a third impurity region1601, an insulating film 1603 formed at the same time as the gateinsulating film, and a capacitive wiring 1604.

In addition, a black mask 1605 is formed on the TFT side of thesubstrate. Note that the capacitive wiring 1604 is formed at the sametime source wiring and drain wiring of the pixel TFT, and iselectrically connected to either ground or to a fixed voltage source.Integration is possible by manufacturing the pixel TFT, the capacitivesection, and the CMOS circuit at the same time on the same substrate. Ofcourse embodiment 13 can be freely combined with any of embodiments 1 to11.

Embodiment 14

An example of embodiment 14, in which the capacitive section thatdiffers from those of embodiments 12 and 13 is formed, is shown in FIG.17. The basic structure is nearly the same as that of embodiment 12, soonly the points of difference are explained. First, a second insulatingfilm 1702 and a black mask 1703, made from a conducting material havinglight shielding properties, are formed. In addition, a third interlayerinsulating film 1704 is formed thereon, and a pixel electrode 1705 isformed out of a transparent conducting film such as ITO or SnO2.

Note that the black mask 1703 covers the pixel TFT section, and moreoverforms a capacitive section with the drain wiring 1701. At this time, thecapacitive section dielectric is the second insulating film 1702. Inaddition, it is possible to have a structure in which a portion of thesecond insulating film 1702 is etched, uncovering a silicon nitride film1706 formed as a passivation film, and then only the silicon nitridefilm 1706 is used as the dielectric.

Therefore, integration is possible by manufacturing the pixel TFT, thecapacitive section, and the CMOS circuit at the same time on the samesubstrate. Of course, embodiment 14 can be freely combined with any ofembodiments 1 to 11.

Embodiment 15

FIG. 18 is used to explain embodiment 15. In embodiment 15 back gateelectrodes 1802 and 1803 are formed below the pixel TFT channel formingregion, through an insulating film 1801. Note that the back gateelectrodes referred to here are electrodes formed with the aim ofcontrolling the threshold voltage and reducing the off current, andpseudo gate electrode is formed on the reverse side to the gate wiring,with the active layer (channel forming region) sandwiched therebetween.

The back gate electrodes 1802 and 1803 can be used with no problems ifthey are made of conducting materials. The present invention involves aheat treatment process at 550 to 650° C. for catalytic elementgettering, so a material with heat resisting properties that can standup to high temperature as above is required. For example, it iseffective to use a silicon gate electrode made from a polysilicon film(either intrinsic or doped with impurities).

In addition, the insulating film 1801 functions as a gate insulatingfilm on the back gate electrode, so that a high quality insulating filmwith few pinholes, etc., is used. A silicon oxide nitride film is usedin embodiment 15, but others such as a silicon oxide film or a siliconnitride film can also be used. However, it is desirable to use amaterial that can realize as flat a face as possible because a TFT willbe manufactured thereon.

By applying a voltage to the back gate electrodes 1802 and 1803 inembodiment 15, the electric field distribution in the channel formingregion is changed, and it is possible to control the threshold voltageand to reduce the off current. This is especially effective for a pixelTFT like the one in embodiment 15.

Note that the structure of embodiment 15 can be freely combined with ofany of embodiments 1 to 14.

Embodiment 16

In embodiment 16 a circuit is constructed with TFTs formed byimplementing the present invention. An example case of the manufactureof an active matrix type liquid crystal display panel formed byintegrating a driver circuit (shift register circuit, buffer circuit,sampling circuit, signal amplification circuit, etc.) with a pixelmatrix circuit on the same substrate is explained.

A CMOS circuit was taken as an example and explained in embodiment 1,but in embodiment 16 a driver circuit with CMOS circuits as basic units,and a pixel matrix circuit with an NTFT as a pixel TFT, are formed onthe same substrate. Note that a multiple gate structure such as a doublegate structure or a triple gate structure may also be used for the pixelTFT.

Note that a structure can be taken in which a pixel electrode is formedto electrically connect the drain wiring, after the pixel TFT on whichthe source wiring and the drain wiring are formed in accordance with theprocesses of embodiment 1. The NTFT structure of the present inventionhas special features, but since it is easy to apply this to a pixel TFTby a known technique, this explanation is omitted.

When a driver circuit and a pixel matrix circuit are formed on the samesubstrate, thereby forming an orientation layer leads to a substantialcompletion of the TFT forming side of the substrate (active matrixsubstrate). Then, if an opposing substrate is prepared with an opposingelectrode and an orientation layer, and if a liquid crystal material isinjected into the space between the active matrix substrate and theopposing substrate, an active matrix type liquid crystal display device(also called liquid crystal display panel or liquid crystal module) withthe structure shown in FIG. 12 is completed. An explanation of theliquid crystal injection process is omitted here because a known cellconstruction process may be used therefor.

Note that in FIG. 12 reference numeral 31 denotes a substrate with aninsulating surface, 32 denotes a pixel matrix circuit, 33 denotes asource driver circuit, 34 denotes a gate driver circuit, 35 denotes anopposing substrate, 36 denotes an FPC (flexible printed circuit), and 37denotes a signal processing circuit such as a D/A converter or a γcompensation circuit. Note that an IC chip is used to form a complexsignal processing circuit, and the IC chip may be mounted on thesubstrate as is the case of COG.

In addition, a liquid crystal display device is given as an example andexplained in embodiment 16, but provided that it is an active matrixtype display device, embodiment 16 can be applied to an EL(electro-luminescence) display panel, an EC (electro-chromatic) displaypanel, an image sensor, and other electro-optical devices.

Further, the electro-optical devices of embodiment 16 can be realized byusing a structure in combination with any of embodiments 1 to 15.

Embodiment 17

It is possible to apply the TFT structure of the present invention toall semiconductor circuits, not just the electro-optical display devicesshown in embodiment 16. Namely, it may be applied to microprocessorssuch RISC processors or ASIC processors, and may be applied in a rangefrom signal processing circuits such as a D/A converter, etc., to thehigh frequency circuits used in portable devices (portable telephones,PHS, mobile computers).

In addition, it is possible to realize a three-dimensional structuresemiconductor device in which a semiconductor circuit using the presentinvention is formed on an interlayer insulating film, which itself hasbeen formed on a conventional MOSFET. It is possible to apply thepresent invention to all semiconductor devices that currently use LSIsin this way. Namely, the present invention may be applied to SOIstructures (TFT structures that use a single crystal semiconductor thinfilm) such as SIMOX, Smart-cut (a registered trademark of SOITECCorporation), ELTRAN (a registered trademark of Canon K.K.), etc.

Further, the semiconductor circuits of embodiment 17 can be realized byusing a structure in combination with any of embodiments 1 to 15

Embodiment 18

A TFT formed by implementing the present invention can be applied tovarious electro-optical devices (embodiment 16) and semiconductorcircuits (embodiment 17). Namely, it is possible to use the presentinvention in all electronic equipments in which electro-optical devicesor semiconductor circuits are incorporated as parts.

The following can be given electronic equipments: video cameras; digitalcameras; projectors; projection televisions; displays used for personalcomputers; displays used for televisions; head mount displays (alsocalled goggle type displays); car navigation systems; image reproductiondevices (DVD players, CD players, MD players, etc.); portableinformation terminals (such as mobile computers, portable telephones,electronic books), etc. Some examples of these are shown in FIGS. 13A to13D, FIGS. 23A to 23D, and FIGS. 24A to 24D.

FIG. 13A is a portable telephone, and is composed of a main body 2001, asound output section 2002, a sound input section 2003, a display device2004, operation switches 2005, and an antenna 2006. The presentinvention can be applied to the sound output section 2002, the soundinput section 2003, the display device 2004, and to other signal controlcircuits.

FIG. 13B is a video camera, and is composed of a main body 2101, adisplay device 2102, a sound input section 2103, operation switches2104, a battery 2105, and an image receiving section 2106. The presentinvention can be applied to the display device 2102, the sound inputsection 2103, and to other signal control circuits.

FIG. 13C is a mobile computer, and is composed of a main body 2201, acamera section 2202, an image receiving section 2203, operating switches2204, and a display device 2205. The present invention can be applied tothe display device 2205, and to other signal control circuits.

FIG. 13D is a goggle type display, and is composed of a main body 2301,a display device 2302, and an arm section 2303. The present inventioncan be applied to the display device 2302, and to other signal controlcircuits.

FIG. 23A is a personal computer, and is composed of a main body 2401, animage input section 2402, a display device 2403, and a keyboard 2404.

FIG. 23B is electronic game equipment such as a television game or avideo game, and is composed of: a main body 2405 loaded with a recordingmedium 2408 and with electric circuits 2412 containing a CPU, etc.; acontroller 2409; a display device 2407; and a display device 2406incorporated into the main body 2405. The display device 2407 and thedisplay device 2406 incorporated into the main body 2405 may bothdisplay the same information, or the former may be taken as the maindisplay and the later may be taken as the sub display to displayinformation from the recording medium 2408, the equipment operationstatus, or touch sensors can be added for use as a control panel.Further, in order for the main body 2405, the controller 2409, and thedisplay device 2407 to communicate with each other, hard wiredcommunication may be used, or sensor sections 2410 and 2411 can beprovided for either wireless communication or optical communication. Thepresent invention can be used in the manufacture of the display devices2406 and 2407. In addition, a conventional CRT can be used for thedisplay device 2407. The present invention can be effectively applied,if the display device 2407 is a 24 to 45 inch liquid crystal television.

FIG. 23C is a player which uses a recording medium on which a program isrecorded (hereafter referred to simply as a recording medium), and iscomposed of a main body 2413, a display device 2414, a speaker section2415, a recording medium 2416, and operation switches 2417. Note that aDVD (digital versatile disk), or CD as a recording medium for thisdevice, and that it can be used for music appreciation, filmappreciation, games, and the Internet. The present invention can beapplied to the display device 2414, and to other signal controlcircuits.

FIG. 23D is a digital camera, and is composed of a main body 2418, adisplay device 2419, an eyepiece section 2420, operation switches 2421,and an image receiving section (not shown). The present invention can beapplied to the display device 2419, and to other signal controlcircuits.

FIG. 24A is a front type projector, and is composed of a display device2601, and a screen 2602. The present invention can be applied to thedisplay device 2601, and to other signal control circuits.

FIG. 24B is a rear type projector, and is composed of a main body 2701,a display device 2702, a mirror 2703, and a screen 2704. The presentinvention may be applied to the display device 2702 (it is especiallyeffective for 50 to 100 inch cases), and to other signal controlcircuits.

FIG. 24C is a drawing showing one example of the structure of thedisplay devices 2601 and 2702 from FIGS. 24A and 24B. The displaydevices 2601 and 2702 consist of an optical light source system 2801,mirrors 2802 and 2805 to 2807, dichroic mirrors 2803 and 2804, opticallenses 2808 and 2809, a liquid crystal display device 2810, a prism2811, and an optical projection system 2812. The optical projectionsystem 2812 is composed of an optical system provided with a projectionlens. Embodiment 18 shows an example in which the liquid crystal displaydevice 2810 is triple stage using three lenses, but there are no speciallimits and a simple stage is acceptable, for example. Further, theoperator may set optical systems such as optical lenses, polarizingfilm, film to regulate the phase difference, IR films, etc., suitablywithin the optical path shown by an arrow in FIG. 24C.

In addition, FIG. 24D shows one example of the structure of the opticallight source system 2801 in FIG. 24C. In embodiment 18 the optical lightsource system 2801 is composed of light sources 2813 and 2814, acompound prism 2815, collimator lenses 2816 and 2820, lens arrays 2817and 2818, and a polarizing conversion element 2819. Note that theoptical light source system shown in FIG. 24D uses two light sources,but three, four, or more light sources, may be used. Of course a singlelight source is acceptable. Further, the operator may set opticallenses, polarizing film, film to regulate the phase difference, IRfilms, etc., suitably in the optical system.

As described above, an applicable range of the present invention isextremely wide, and it can be applied to electronic equipment in allfields. Further, the electronic equipments of embodiment 18 can berealized by using a structure in combination with any of embodiments 1to 17.

Embodiment 19

FIGS. 21A to 21E are used to explain the manufacturing process of a CMOScircuit with a different structure from the one in embodiment 1. Notethat the processing is nearly the same as in embodiment 1 until anintermediate step, so only the difference is explained.

First, processing is performed in accordance with embodiment 1 until itreaches the processes of FIG. 3D. However, the technique of JapanesePatent Application Laid-open No. Hei 7-130652 is used in embodiment 1during formation of the active layers 303 and 304, while in embodiment19 a crystallization example that does not use a catalytic element isshown.

In embodiment 19, after a 50 nm thick amorphous silicon is formed by CVDor sputtering, it is crystallized by irradiation of tight from anexcimer laser, with KrF as the excitation gas. Of course an excimerlaser using XeCl as the excitation gas, and the third or fourth harmoniccomponent of a Nd:YAG laser may also be used. In addition, throughputcan be effectively increased by setting the laser light cross section toa linear one.

Note that a polysilicon film is obtained by laser crystallization of aninitial film as an amorphous silicon film in embodiment 19, but it isalso acceptable to use a micro-crystalline silicon film as the initialfilm, and to directly deposit a polysilicon film. Of course laserannealing may be performed on a deposited polysilicon film.

Further, furnace annealing may be substituted for laser annealing.Namely, crystallization may be carried out by annealing in an electricfurnace at about 600° C.

An amorphous film is crystallized by generation of a natural nucleus inembodiment 19, and the polysilicon film formed in this way is then usedto form the active layers 303 and 304. Other processes are performed inaccordance with embodiment 1, and the state in FIG. 3D is then obtained.

Next, a resist mask 401 that covers a portion of the NTFT, and a resistmask 402 that covers all of the PTFT, are formed as shown in FIG. 21A.Then the gate insulating film 305 shown in FIG. 3A is dry etched in thisstate, forming the gate insulating film 403.

At this point the length of the portion of the date insulating film 403that protrudes outward beyond the sidewall 312 (the length of theportion of the gate insulating film 403 that contacts the secondimpurity region 314) determines by the length (width) of the secondimpurity region 104, as shown in FIG. 1. Therefore, it is necessary toprecisely perform the mask alignment of the resist mask 316.

A third phosphorous doping process is performed after obtaining thestate in FIG. 21A. This time the acceleration voltage is set low at 10KeV because phosphorous is doped into an exposed active layer. Note thatthe dose is regulated to obtain a phosphorous concentration of 5×10²⁰atoms/cm³ contained in a third impurity region 404 formed in this way.The phosphorous concentration at this time is denote (n⁺). (See FIG.21B.)

Phosphorous is not doped into the area shielded by the resist mask 401in this process, so the second impurity region 314 remains as is in thatarea. Therefore the second impurity region 104 shown in FIG. 1 isdefined here. At the same time, the third impurity region 105 shown inFIG. 1 is defined.

The second impurity region 314 functions as a 2nd LDD region, and thethird impurity region 404 functions as a source region or a drainregion.

Next, the resist masks 401 and 402 are removed, and a resist mask 406that newly covers all of the NTFT is formed. Then the sidewall 313 isremoved, and in addition a gate insulating film 407 is formed by dryetching the gate insulating film 305 into the same shape as the gateinsulating film 307. (See FIG. 21C.)

A boron doping process is performed after obtaining the state in FIG.21C. The acceleration voltage is set to 10 KeV here, and the dose isregulated to obtain a boron concentration of 3×10²⁰ atoms/cm³ containedin a fourth impurity region 408 formed in this way. The boronconcentration at this time is denoted (p⁺⁺). (See FIG. 21D.)

At this time, the channel forming region 311 is formed on the inside ofthe gate wiring 307 because boron also wraps around and is doped on theinside of the gate wiring 307. Further, the first impurity region 309and the second impurity region 315 formed on the PTFT side are invertedinto p-type by boron in this process. Therefore, the resistance valuesin what were originally the first impurity region and the secondimpurity region change, but this is not a problem because boron is dopedinto a sufficiently high concentration.

This defines the fourth impurity region 110 shown in FIG. 1. The fourthimpurity region 408 is formed in a completely self-aligning manner byusing the gate wiring 307 as a mask, and functions as a source region ora drain region. Neither LDD region nor offset region is formed on thePTFT in embodiment 19, but this is no problem because PTFTs inherentlyhave high reliability, and on the contrary, by not forming an LDDregion, the on current an be gained, so there are cases when this isconvenient.

This leads finally to an NTFT active layer in which a channel formingregion, a first impurity region, a second impurity region, and a thirdimpurity region are formed, and to a PTFT active layer in which achannel forming region and a fourth impurity region are formed, as shownin FIG. 21D.

After obtaining the state in FIG. 21D, a first insulating film 409 isformed to a thickness of 1 μm. A silicon oxide film, a silicon nitridefilm, an oxide silicon nitride film (an insulating film expressed bySiOxNy), an organic resin film, or a laminate of these films can be usedas the first insulating film 409. An acrylic resin film is employed inembodiment 19.

Source wirings 410 and 411, and a drain wiring 412 are formed ofmetallic materials after the first insulating film 409 is formed. Athree layer laminate wiring of an aluminum film which contains titanium,sandwiched between titanium films, is used in embodiment 19.

Further, if a so-called BCB (benzocyclobutane) resin film is used forthe first insulating film 409, then the flatness is increased and at thesame time it becomes possible to use copper for a wiring material.Copper is extremely effective as a wiring material because the wiringresistance is low.

A silicon nitride film 413 is formed as a passivation film to athickness of 50 nm after the source wiring and the drain wiring areformed. In addition a second interlayer insulating film 414 is formedthereon as a protective film. It is possible to use the same materialsfor the second interlayer insulating film 414 as those given above forthe first insulating film 409. A laminate structure of an acrylic resinfilm on a 50 nm thick silicon oxide film is employed in embodiment 19.

A CMOS circuit with a structure as shown in FIG. 21E can be completed byfollowing the above processes. The CMOS circuit formed in accordancewith embodiment 19 has a remarkably improved reliability for the entirecircuit because the NTFT possesses outstanding reliability. Further,using a structure of embodiment 19, the balance of characterstics (thebalance of electrical characteristic) between the NTFT and the PTFTbecomes good, so it becomes difficult to cause malfunction.

Note that it is possible to freely combine embodiment 19 with thestructure of any of embodiments 2, 3, and 9 to 15. Embodiment 19 is alsoapplicable to the structure of any of embodiments 16 to 18.

Embodiment 20

This embodiment demonstrates a process for producing an EL(electroluminescence) display device according to the invention of thepresent application.

FIG. 25A is a top view showing an EL display device which was producedaccording to the invention of the present application. In FIG. 25A,there are shown a substrate 4010, a pixel portion 4011, a source sidedriving circuit 4012, a gate side driving circuit 4013, each drivingcircuit connecting to wirings 4014-4016 which reach FPC 4017 leading toexternal equipment.

The pixel portion, preferably together with the driving circuit, isenclosed by a sealing material (or housing material) 4018. The sealingmaterial 4018 may be a concave metal plate or glass plate which enclosesthe element; alternatively, it may be an ultraviolet curable resin. Aconcave metal plate should be fixed to the substrate 4010 with anadhesive 4019 so that an airtight space is formed between the metalplate and the substrate 4010. Thus, the EL element is completely sealedin the airtight space and completely isolated from the outside air.

It is desirable that the cavity 4020 between the sealing material 4018an the substrate 4010 be filled with an inert gas (such as argon,helium, and nitrogen) or a desiccant (such as barium oxide), so as toprotect the EL element from degradation by moisture.

FIG. 25B is a sectional view showing the structure of the EL displaydevice in this Embodiment. There is shown a substrate 4010, anunderlying coating 4021, a TFT 4022 for the driving circuit, and a TFT4023 for the pixel portion. (The TFT 4022 shown is a CMOS circuitconsisting of an n-channel type TFT and a p-channel type TFT. The TFT4023 shown is the one which controls current to the EL element.) As aTFT 4022 fir the driving circuit, the NTFT and PTFT shown in FIG. 1 canbe employed. As a TFT 4023 for the pixel portion, an NTFT or a PTFT canbe employed.

Upon completion of TFT 4022 (for the driving circuit) and TFT 4023 (forthe pixel portion), a pixel electrode 4027 is formed on the interlayerinsulating film (a leveling film) 4026 made of a resin. This pixelelectrode is a transparent conductive film which is electricallyconnected to the drain of TFT 4023 for the pixel portion. Thetransparent conductive film may be formed from a compound (called ITO)of indium oxide and tin oxide or a compound of indium oxide and zincoxide. An insulating film 4028 is formed on the pixel electrode 4027,and an opening is formed above the pixel electrode 4027.

Subsequently, the EL layer 4029 is formed. It may be of single-layerstructure or multi-layer structure by freely combining known ELmaterials such as a hole injection layer, a hole transport layer, alight emitting layer, an electron transport layer, and an electroninjection layer. Any known technology may be available for suchstructure. The EL material is either a low-molecular material or ahigh-molecular material (polymer). The former may be applied by vapordeposition, and the latter may be applied by a simple method such asspin coating, printing, or ink-jet method.

In this embodiment, the EL layer is formed by vapor deposition through ashadow mask. The resulting EL layer permits each pixel to emit lightdiffering in wavelength bred emitting layer, green emitting layer, andblue emitting layer). This realizes the color display. Alternativesystems available include the combination of color conversion layer(CCM) and color filter and the combination of white light emitting layerand color filter. Needless to say, the EL display device may bemonochromatic.

On the EL layer is formed a cathode 4030. Prior to this step, it isdesirable to clear moisture and oxygen as much as possible from theinterface between the EL layer 4029 and the cathode 4030. This objectmay be achieved by forming the EL layer 4029 and the cathode 4030consecutively in a vacuum, or by forming the EL layer 4029 in an inertatmosphere and then forming the cathode 4030 in the same atmospherewithout being exposed to air into it. In this Embodiment, the desiredfilm was formed by using a film-forming apparatus of multi-chambersystem (cluster tool system).

The multi-layer structure composed of lithium fluoride film and aluminumfilm is used in this Embodiment as the cathode 4030. To be concrete, theEL layer 429 is coated by vapor deposition with a lithium fluoride film(1 nm thick) and an aluminum film (300 nm thick) sequentially. Needlessto say, the cathode 4030 may be formed from MgAg electrode which is aknown cathode material. Subsequently, the cathode 4030 is connected to awiring 4016 in the region 4031. The wiring 4016 to supply a prescribedvoltage to the cathode 4030 is connected to the FPC 4017 through anelectrically conductive paste material 4032.

The electrical connection between the cathode 4030 and the wiring 4016in the region 4031 needs contact holes in the interlayer insulating film4026 and the insulating film 4028. These contact holes may be formedwhen the interlayer insulating film 4026 undergoes etching to form thecontact hole for the pixel electrode or when the insulating film 4028undergoes etching to form the opening before the EL layer is formed.When the insulating film 4028 undergoes etching, the interlayerinsulating film 4026 may be etched simultaneously. Contact holes of goodshape may be formed if the interlayer insulating film 4026 and theinsulating film 4028 are made of the same material.

The wiring 4016 is electrically connected to FPC 4017 through the gap(filled with an adhesive 4019) between the sealing material 4018 and thesubstrate 4010. As in the wiring 4016 explained above, other wirings4014 and 4015 are also electrically connected to FPC 4017 under thesealing material 18.

The invention can apply to the EL display panel having the constitutionas above. The structure of the pixel region in the panel is illustratedin more detail. FIG. 26 shows the cross section of the pixel region;FIG. 27A shows the top view thereof; and FIG. 27B shows the circuitstructure for the pixel portion. In FIG. 26, FIG. 27A and FIG. 27B, thesame reference numerals are referred to for the same parts, as beingcommon thereto.

In FIG. 26, the switching TFT 4102 formed on the substrate 4101 is anNTFT of the invention. In this Embodiment, it has a double-gatestructure, but its structure and fabrication process do not so muchdiffer from the structures and the fabrication processes illustratedherein above, and their description is omitted herein. However, thedouble-gate structure of the switching TFT 4102 has substantially twoTFTs as connected in series, and therefore has the advantage of reducingthe off-current to pass therethrough. In this Embodiment, the switchingTFT 4102 has such a double-gate structure, but is not limitative. It mayhave a single-gate structure or a triple-gate structure, or even anyother multi-gate structure having more than three gates. As the case maybe, the switching TFT 4102 may be a PTFT of the invention.

The current-control TFT 4103 is an NTFT of the invention. The drainwiring 4135 in the switching TFT 4102 is electrically connected with thegate electrode 4137 in the current-control TFT, via the wiring 4136therebetween. The wiring indicated by 4138 is a gate wiring forelectrically connecting the gate electrodes 4139 a and 4139 b in theswitching TFT 4102

It is very important that the current-control TFT 4103 has the structuredefined in the invention. The current-control TFT is an element forcontrolling the quantity of current that passes through the EL device.Therefore, a large quantity of current passes through it, and thecurrent-control TFT has a high risk of thermal degradation anddegradation with hot carriers. Therefore, the structure of the inventionis extremely favorable, in which an LDD region is so constructed thatthe gate electrode (strictly, the side wall functioning as the gateelectrode) overlaps with the drain region in the current-control TFT,through a gate insulating film therebetween.

In this Embodiment, the current-control TFT 4103 is illustrated to havea single-gate structure, but it may have a multi-gate structure with aplurality of TFTs connected in series. In addition, plural TFTs may beconnected in parallel so that the channel-forming region issubstantially divided into plural sections. In the structure of thattype, heat radiation can be effected efficiently. The structure isadvantageous for protecting the device with it from thermaldeterioration.

As in FIG. 27A, the wiring to be the gate electrode 4137 in thecurrent-control TFT 4103 overlaps with the drain wiring 4140 therein inthe region indicated by 4104, via an insulating film therebetween. Inthis state, the region indicated by 4104 form a capacitor. The capacitor4104 functions to retain the voltage applied to the gate in thecurrent-control TFT 4103. The drain wiring 4140 is connected with thecurrent supply line (power line) 4201, from which a constant voltage isall the time applied to the drain wiring 4140.

On the switching TFT 4102 and the current-control TFT 4103, formed is afirst passivation film 4141. On the film 4141, formed is a leveling film4142 of an insulating resin. It is extremely important that thedifference in level of the layered parts in TFT is removed throughplanarization with the leveling film 4142. This is because the EL layerto be formed on the previously formed layers in the later step isextremely thin, and if there exist a difference in level of thepreviously formed layers, the EL device will be often troubled by lightemission failure. Accordingly, it is desirable to previously planarizeas much as possible the previously formed layers before the formation ofthe pixel electrode thereon so that the EL layer could be formed on theleveled surface.

The reference numeral 4143 indicates a pixel electrode (a cathode in theEL device) of a conductive film with high reflectivity. The pixelelectrode 4143 is electrically connected with the drain region in thecurrent-control TFT 4103 It is preferable that the pixel electrode 4143is of a low-resistance conductive film of an aluminum alloy, a copperalloy or a silver alloy, or of a laminate of those films.Needless-to-say, the pixel electrode 4143 may have a laminate structurewith any other conductive films.

In the recess (this corresponds to the pixel) formed between the banks4144 a and 4144 b of an insulating film (preferably of a resin), thelight-emitting layer 4145 is formed. In the illustrated structure, onlyone pixel is shown, but plural light-emitting layers could be separatelyformed in different pixels, corresponding to different colors of R(red), G (green) and B (blue). The organic EL material for thelight-emitting, layer may be any π-conjugated polymer material. Typicalpolymer materials usable herein include polyparaphenylenevinylene (PVV)materials, polyvinylcarbazole (PVK) materials, polyfluorene materials,etc.

Various types of PVV-type organic EL materials are known, such as thosedisclosed in “H. Shenk, H. Becker, O. Gelsen, E. Klunge, W. Kreuder, andH. Spreitzer; Polymers for Light Emitting Diodes, Euro DisplayProceedings, 1999, pp. 33-37” and in Japanese Patent Laid-Open No.10-92576. Any of such known materials are usable herein.

Concretely, cyanopolyphenylenevinylenes may be used for red-emittinglayers; polyphenylenevinylenes may be for green-emitting layers; andpolyphenylenevinylenes or polyalkylphenylenes may be for blue-emittinglayers. The thickness of the film for the light-emitting layers may fallbetween 30 and 150 nm (preferably between 40 and 100 nm).

These compounds mentioned above are referred to merely for examples oforganic EL materials employable herein and are not limitative at all.The light-emitting layer may be combined with a charge transportationlayer or a charge injection layer in any desired manner to form the ELlayer (a layer for light emission and for carrier transfer for lightemission).

Specifically, this Embodiment is to demonstrate the embodiment of usingpolymer materials to form light-emitting layers, which, however, is notlimitative. Apart from this, low-molecular organic EL materials may alsobe used for light-emitting layers. For charge transportation layers andcharge injection layers, further employable are inorganic materials suchas silicon carbide, etc. Various organic EL materials and inorganicmaterials for those layers are known, any of which are usable herein.

In this Embodiment, a hole injection layer 4146 of PEDOT (polythiophene)or PAni (polyaniline) is formed on the light-emitting layer 4145 to givea laminate structure for the EL layer. On the hole injection layer 4146,formed is an anode 4147 of a transparent conductive film. In thisEmbodiment, the light having been emitted by the light-emitting layer4145 radiates therefrom in the direction toward the top surface (thatis, in the upward direction of TFT). Therefore, in this, the anode musttransmit light. For the transparent conductive film for the anode,usable are compounds of indium oxide and tin oxide, and compounds ofindium oxide and zinc oxide. However, since the anode is formed afterthe light-emitting layer and the hole injection layer having poor heatresistance have been formed, it is preferable that the transparentconductive film for the anode is of a material capable of being formedinto a film at as low as possible temperatures.

When the anode 4147 is formed, the EL device 4105 is finished. The ELdevice 4105 thus fabricated herein indicates a capacitor comprising thepixel electrode (cathode) 4143, the light-emitting layer 4145, the holeinjection layer 4146 and the anode 4147. As in FIG. 27A, the region ofthe pixel electrode 4143 is nearly the same as the area of the pixel.Therefore, in this, the entire pixel portion functions as the EL device.Accordingly, the light utility efficiency of the EL device fabricatedherein is high, and the device can display bright images.

In this Embodiment, a second passivation film 4148 is formed on theanode 4147. For the second passivation film 4148, preferably used is asilicon nitride film or a silicon oxynitride film. The object of thesecond passivation film 4148 is to insulate the EL device from theoutward environment. The second passivation film 4148 has the functionof preventing the organic EL material from being degraded throughoxidation and has the function of preventing it from degassing. With thesecond passivation film 4148 of that type, the reliability of the ELdisplay device is improved.

As described herein above, the EL display panel of the inventionfabricated in this Embodiment has a pixel portion for the pixel portionhaving the constitution as in FIG. 26, and has the switching TFT throughwhich the off-current to pass is very small to a satisfactory degree,and the current-control TFT resistant to hot carrier injection.Accordingly, the EL display panel fabricated herein has high reliabilityand can display good images.

The constitution of this Embodiment can be combined with anyconstitution of Embodiments 2 to 13, 15 or 19 in any desired manner.Incorporating the EL display panel of this Embodiment into theelectronic appliance of Embodiment 18 as its display part isadvantageous.

Embodiment 21

This Embodiment is to demonstrate a modification of the EL display panelof Embodiment 20, in which the EL device 4105 in the pixel portion has areversed structure. For this Embodiment, referred to is FIG. 28. Theconstitution of the EL display panel of this Embodiment differs fromthat illustrated in FIG. 27A only in the EL device part and thecurrent-control TFT portion. Therefore, the description of the otherportions except those different portions is omitted herein.

In FIG. 28, the current-control TFT 4301 may be PTFT of the invention.For the process of forming it, referred to is that of Embodiment 1.

In this Embodiment, the pixel electrode (anode) 4150 is of a transparentconductive film. Concretely, used is a conductive film of a compound ofindium oxide and zinc oxide. Needless-to-say, also usable is aconductive film of a compound of indium oxide and tin oxide.

After the banks 4151 a and 4151 b of an insulating film have beenformed, a light-emitting layer 4152 of polyvinylcarbazole is formedbetween them in a solution coating method. On the light-emitting layer4152, formed are an electron injection layer 4153 of acetylacetonatepotassium, and a cathode 4154 of an aluminum alloy. In this case, thecathode 4154 serves also as a passivation film. Thus is fabricated theEL device 4302.

In this Embodiment, the light having been emitted by the light-emittinglayer radiates in the direction toward the substrate with TFT formedthereon, as in the direction of the arrow illustrated. In the case ofthe constitution of this Embodiment, it is preferable that thecurrent-control TFT 2601 is PTFT.

The constitution of this Embodiment can be combined with anyconstitution of Embodiments 2 to 13, 15 or 19 in any desired manner.Incorporating the EL display panel of this Embodiment into theelectronic appliance of Embodiment 18 as its display part isadvantageous.

Embodiment 22

This Embodiment is to demonstrate modifications of the pixel with thecircuit structure of FIG. 27B. The modifications are as in FIG. 29A toFIG. 29C. In this Embodiment illustrated in those FIG. 29A to FIG. 29C,5001 indicates the source wiring for the switching TFT 5002; 5003indicates the gate wiring for the switching TFT 5002; 5004 indicates acurrent-control TFT; 5005 indicates a capacitor; 5006 and 5008 indicatecurrent supply lines; and 5007 indicates an EL device.

In the embodiment of FIG. 29A, the current supply line 5006 is common tothe two pixels. Specifically, this embodiment is characterized in thattwo pixels are lineal-symmetrically formed with the current supply line5006 being the center between them. Since the number of current supplylines can be reduced therein, this embodiment is advantageous in thatthe pixel portion can be much finer and thinner.

In the embodiment of FIG. 29B, the current supply line 5008 is formed inparallel to the gate wiring 5003. Specifically, in this, the currentsupply line 5008 is so constructed that it does not overlap with thegate wiring 5003, but is not limitative. Being different from theillustrated case, the two may overlap with each other via an insulatingfilm therebetween so far as they are of different layers. Since thecurrent supply line 5008 and the gate wiring 5003 may enjoy the commonexclusive area therein, this embodiment is advantageous in that thepixel portion can be much finer and thinner.

The structure of the embodiment of FIG. 29C is characterized in that thecurrent supply line 5008 is formed in parallel to the gate wings 5003,like in FIG. 29B, and that two pixels are lineal-symmetrically formedwith the current supply line 5008 being the center between them. Inthis, it is also effective to provide the current supply line 5008 insuch a manner that it overlaps with any one of the gate wirings 5003.Since the number of current supply lines can be reduced therein, thisembodiment is advantageous in that the pixel pattern can be much finerand thinner.

The constitution of this Embodiment can be combined with anyconstitution of Embodiment 20 or 21 in any desired manner. Incorporatingthe EL display panel having the pixel portion of this Embodiment intothe electronic equipments of Embodiment 18 as its display portion isadvantageous.

Embodiment 23

The embodiment of Embodiment 20 illustrated in FIG. 27A and FIG. 27B isprovided with the capacitor 4104 which acts to retain the voltageapplied to the gate electrode in the current-control TFT 4103. In theembodiment, however, the capacitor 4104 may be omitted.

In the embodiment of Embodiment 20, the current-control TFT 4103 is NTFTof the invention, as in FIG. 26. Therefore, in the embodiment, the LDDregion is so formed that it overlaps with the gate electrode (strictly,the side wall) through the gate insulating film therebetween. In theoverlapped region, formed is a parasitic capacitance generally referredto as a gate capacitance. This Embodiment is characterized in that theparasitic capacitance is positively utilized in place of the capacitor4104.

The parasitic capacitance varies, depending on the area in which theside wall overlaps with the LDD region, and is therefore determinedaccording to the length of the LDD region in the overlapped area.

Also in the embodiments of Embodiment 22 illustrated in FIG. 29A, FIG.29B and FIG. 29C, the capacitor 5005 can be omitted.

The constitution of this Embodiment can be combined with anyconstitution of Embodiment 20 or 21 in any desired manner. Incorporatingthe EL display panel having the pixel structure of this Embodiment intothe electronic equipments of Embodiment 18 as its display part isadvantageous.

Embodiment 24

In Embodiment 24, another EL module is explained, as shown in FIGS. 30Aand 30B. The same reference numerals in FIGS. 30A and 30B as in FIGS.25-29C indicate same constitutive elements, so an explanation isomitted.

FIG. 30A shows a top view of the EL module in this embodiment and FIG.30B shows a sectional view of A-A′ of FIG. 30A. A filling material 6004,a covering material 6000, a sealing material 6002, a flame material 6001and a passivation film 6003 are formed in the EL module.

According to Embodiment 20, a structure until a cathode 4030 of an ELelement is fabricated. Further, a passivation film 6003 is formed tocover a surface of the EL element.

A filling material 6004 is formed to cover the EL element and alsofunctions as an adhesive to adhere to the covering material 6000. As thefilling material 6004, PVC (polyvinyl chloride), an epoxy resin, asilicon resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate)can be utilized. It is preferable to form a desiccant in the fillingmaterial 6004, since a moisture can be maintained.

As a covering material 6001, a glass plate, an aluminum plate, astainless plate, a FRP (Fiberglass-Reinforced Plastics) plate, a PVF(polyvinyl fluoride) film, a Mylar film, a polyester film or an acrylfilm can be used. In a case that PVB or EVA is employed as the fillingmaterial 6004, it is preferable to use an aluminum foil with a thicknessof some tens of μm sandwiched by a PVF film or a Mylar film.

It is noted that the covering material 6000 should have a lighttransparency with accordance to a light emitting direction (a lightradiation direction) from the EL element.

Next, the covering material 6000 is adhered using the filling material6004. Then, the flame material 6001 is attached to cover side portions(exposed faces) of the filling material 6004. The flame material 6001 isadhered by the sealing material (acts as an adhesive) 6002. As thesealing material 6002, a light curable resin is preferable. Also, athermal curable resin can be employed if a heat resistance of the ELlayer is admitted. It is preferable for the sealing material 6002 not topass moisture and oxygen. In addition it is possible to add a desiccantinside the sealing material 6002.

Embodiment 25

In Embodiment 25, a different EL module from Embodiments 20-24 isexplained, as shown in FIGS. 31A and 31B. The same reference numerals inFIGS. 31A and 31B as in FIGS. 25-30A indicate same constitutiveelements, so an explanation is omitted.

FIG. 31A shows a top view of the EL module in this embodiment and FIG.31B shows a sectional view of A-A′ of FIG. 31A.

In Embodiment 25, only the difference between Embodiment 24 and the thisembodiment is explained. After attaching a covering material 6000, aflame material 6001 is formed in Embodiment 24. On the other hand, asshown in FIG. 31B, a sealing material 7000 is formed to locate insidethe substrate 4010 and the covering material 6000. Further, anothersealing material 7000 covers outside thereof. The second sealingmaterial 7001 also covers FPC 4017 to seal inside the EL element.

It is possible to raise the reliability of an NTFT by implementing thepresent invention. Therefore, it is possible to ensure the reliabilityof an NTFT having high electrical characteristics (especially highmobility) that are required for strict reliability. At the same time, byforming a CMOS circuit with an NTFT and a PTFT that have a superiorbalance of characteristic, a semiconductor circuit showing highreliability and outstanding electrical characteristics can be formed.

In addition, semiconductor devices having few instability factors can berealized because it is possible to reduce the catalytic element used tocrystallize the semiconductor according to the present invention.Moreover, there is no reduction in throughput because the process thatreduces the catalytic element is performed at the same time as theformation and activation of the source region and the drain region.

Furthermore, by raising the reliability of circuits constructed by TFTs,as above, it is possible to ensure the reliability of all semiconductordevices, including electro-optical devices, semiconductor circuits, andelectronic equipments.

1. A light emitting device comprising: a substrate; a gate signal lineover the substrate; a power source supply line over the substrate; afirst thin film transistor over the substrate, the first thin filmtransistor electrically connected to the gate signal line; a second thinfilm transistor over the substrate, the second thin film transistorelectrically connected to the power source supply line; an interlayerinsulating film over the first thin film transistor and the second thinfilm transistor, a first electrode over the interlayer insulating film,the first electrode electrically connected to the second thin filmtransistor through a contact hole formed in the interlayer insulatingfilm; and a light emitting layer disposed adjacent to a side edge of abank, the light emitting layer formed over the first electrode, and asecond electrode over the light emitting layer, wherein the lightemitting layer has a flattened upper surface over the contact hole,wherein the light emitting layer does not extend to an upper surface ofthe bank, and wherein the gate signal line and the power source supplyline are parallel to each other.
 2. The light emitting device accordingto claim 1, further comprising; a cover over the second electrode,wherein a gap between the second electrode and the cover is filled withan inert gas.
 3. The light emitting device according to claim 1, whereinthe first thin film transistor has a multi-gate structure, and whereinan LDD region is formed in a semiconductor film of the second thin filmtransistor, and wherein the LDD region overlaps with a gate electrode ofthe second thin film transistor.
 4. The light emitting device accordingto claim 1, wherein the light emitting layer includes a polymermaterial.
 5. The light emitting device according to claim 1, wherein thelight emitting device is incorporated into an electronic equipmentselected from the group consisting of a video camera, a digital camera,a projector, a television, a head mount display, a car navigationsystem, an image reproduction device, a portable telephone, a computer,a goggle type display, and an electronic game equipment.
 6. A lightemitting device comprising: a substrate; a gate signal line over thesubstrate; a power source supply line over the substrate; a first thinfilm transistor over the substrate, the first thin film transistorelectrically connected to the gate signal line; a second thin filmtransistor over the substrate, the second thin film transistorelectrically connected to the power source supply line; an interlayerinsulating film over the first thin film transistor and the second thinfilm transistor; a first electrode over the interlayer insulating film,the first electrode electrically connected to the second thin filmtransistor through a contact hole formed in the interlayer insulatingfilm; and a light emitting layer disposed adjacent to a side edge of abank, the light emitting layer formed over the first electrode, and asecond electrode over the light emitting layer, wherein the lightemitting layer has a flattened upper surface over the contact hole,wherein the light emitting layer does not extend to an upper surface ofthe bank, wherein the second electrode extends to the upper surface ofthe bank, and wherein the gate signal line and the power source supplyline are parallel to each other.
 7. The light emitting device accordingto claim 6, further comprising; a cover over the second electrode,wherein a gap between the second electrode and the cover is filled withan inert gas.
 8. The light emitting device according to claim 6, whereinthe first thin film transistor has a multi-gate structure, and whereinan LDD region is formed in a semiconductor film of the second thin filmtransistor, and wherein the LDD region overlaps with a gate electrode ofthe second thin film transistor.
 9. The light emitting device accordingto claim 6, wherein the light emitting layer includes a polymermaterial.
 10. The light emitting device according to claim 6, whereinthe light emitting device is incorporated into an electronic equipmentselected from the group consisting of a video camera, a digital camera,a projector, a television, a head mount display, a car navigationsystem, an image reproduction device, a portable telephone, a computer,a goggle type display, and an electronic game equipment.